Unfortunately, AT isn't available until level 9 at the earliest without some early-entry shenanigans, which makes it a decent fleshing PrC to round off level 20, but isn't available for entry until the character is already taking levels of Daggerspell Mage.

Actually, it was an argument from the WoTC community site. Found it again, and this is the FAQ response: I must have got that Wiz/Sor thing from another forum. I've gone through about seven or so :P I really don't think AT is something that will mesh well with my character anyway. It's main bonus seems to be the ability to use a skill at a distance, which is something I don't see someone needing to do too often if ever, and it's only other major benefits are 5 SA dice, and full spell progression. It really seems more like a mage class that can do rogue skill than a rogue that can boost it's abilities through magic.

Re-reading the DSM, it does remind me of a duskblade, but focused on daggers and using sneak attacks. So, semi off topic question: How well do you guys think a spellthief/duskblade combo would work? It's a possible swap in or back up character idea of mine that could use some fleshing out.

Like I said, Spellthief 1/Duskblade 5. I'd go something like:

Human - Weapon Finesse
Spellthief 1 - Two-Weapon Fighting
Duskblade 1
Duskblade 2 - Master Spellthief
Duskblade 3
Duskblade 4
Duskblade 5 - Weapon Focus (Dagger)
Daggerspell Mage 1
Daggerspell Mage 2
Daggerspell Mage 3 - Craven
etc.

I had completely forgotten about the master spellthief feat, major slip up on my part. I figure you recommend going full DSM with this, but what about the last 4 levels? Duskblade might give me more BAB, but not too much else. Maybe spellthief's a better choice, as you can nab the extra SA dice, plus a few more spell stealing abilites (steal effect, spell-like ability, etc,) the spellgrace bonus, at the cost of a slightly lower BAB.

I'd suggest Abjurant Champion usually, but Duskblade doesn't really mesh well with that PrC due to its lack of standard abjuration spells. Arcane Trickster is still an option because it grants more SA dice and full spell progression. Spellsword is an option, though not a very good one with only half spell progression, and jack-all for class features.

If you were playing an evil character, you could go into Assassin.

You'll lose one BAB in the process, but your best option is probably Unseen Seer.

I'll look into the Unseen seer more. Not going with an evil character due to possible party conflictions (though I think it's pretty stupid you have to be evil to be an assassin :/) Maybe if the DM allows for neutral aligned assassins.

Well, if the DM allows evil, a simple Ring of Mind Shielding can take care of most of your "evil character party conflicts" problems, especially if you play a Lawful Evil who simply does his job quietly and doesn't play some mustache-twirling stereotype. I played a Lawful Evil Rogue/Assassin in a party once, the only evil member of the group, and no one, IC or OOC, realized I was actually evil until I told them. Myself and the DM worked together so all of my actual contracted assassinations happened during downtime between sessions, and I was careful not to leave evidence behind for the authorities or my teammates to know it was me doing the killings. Only time it caught me in the butt was when I missed a Banshee's Wail trap, failed my Fort save, and died... the cleric couldn't resurrect me because his god wouldn't allow him to bring back an evil person from the dead (a situation I knew had the possibility of happening and agreed to before making my evil character).

Now that sounds like one well played character. I do keep evil characters as a possible option every now and then, even played one once. He actually started out lawful neutral, but turned neutral evil as a result of a growing hatred for his companions' buffoonery and making a deal with an efreeti.

Well, the Unseen Seer does indeed look the best. My only issue is that 4th level and how it cripples the other arcane spells I'd know in favor of divination. I could always take that last level as Spellthief or Duskblade I suppose, which does remind me of a question: Should I use my prestige levels to boost my ST spells, or DB spells? DB has more melee ones to choose from, but ST has the option of picking from a handful of schools. Only downside to the ST is that I'd burn a couple prestige levels to gain access to the ST's versatility.

Unseen Seer is hands down the best arcane/sneaky class, and really, the best spellcasting/sneaky combo prestige class in the game full stop.

The Divination Mastery thing doesn't 'cripple' your spells, it gives them -1 CL, which is counteracted by practiced spellcaster, so in reality, given that you're going to have practiced spellcaster ANYWAY to offset your rogue levels, you basically just net a free +1 divination.

As for duskblade spellthief etc, i'd really recommend rogue/wizard. Duskblade is kind of okay at low levels but it very very quickly loses puff and it has few of the utility spells that can really make a rogue shine. When you have the BAB to qualify, levels in Abjurant Champion are a good idea as well. Dipping swordsage gets you access to Shadow Blade and Assassin's Stance which help your sneak attack damage, as well as various utility maneuvers including going invisible for a round (!!) and being able to avoid full attack and charges. Typically take it one level before you get to prestige out, as the higher level you take it the better but you don't want to delay your prestiges too long.