Light shields and shield proficiency Qs

[Sinth]

As a rogue using a light shield (0 base armor penalty) will I incur penalties to my attacks or skill checks if I do not have the shield proficiency feat?

Also, what are the actual penalties for unproficient shield use?

I already know if would recieve penalties if it were a tower shield, but what about light wooden / bucklers / heavy shields ?

Basically I like my rogue to have high AC because I'm still new. I get aggro alot from my sneak attacks so its good to be able to defend and let the aggro cool down. So would I be wasting a feat by getting shield proficiency?
I am planning on going pure rogue and currently Mechanic spec'd and almost totally geared towards Disabling Devices / Search / Open Lock / UMD.

Thanks in advance for any help. Yes, I know my damage sucks without TWF, but like I said I'm going pure skills for now.

[PurdueDave]

You can use a zero base penalty shield, like a light mithral shield, with no downside.

[The_Cataclysm]

The best shields you can get for a rogue are mithril heavy shield. Best AC with no penalty.

[Master_of_None]

Instead of spending a feat on shield proficiency, I think you'd be better off taking at least one level of fighter or ranger. I know you said you were planning on going pure rogue, but in my opinion, there's virtually no reason not to take one level of another class in the game as it is now. Yes, you can use some shields without penalty as the above posters have suggested, but you can gain a lot from a level of fighter (two levels is probably better for ranger or paladin), and lose almost nothing from your rogue abilities.

I would also suggest spending some time on the rogue class forum. The kind of build you're suggesting, focused on skills, is not going to be in much demand at higher levels. Most people don't care much about traps in higher levels quests (with some notable exceptions), and if they want rogues at all it's for their combat abilities. You don't necessarily have to go twf, but if you don't plan out how you're going to contribute to combat, you could find it difficult to find groups later in the game.

[Delzon]

If you are not proficient with a shield you take the shield's armor check penalty on attack rolls, and all skil checks that involve moving. They are suggesting a mithral shield because it drops the armor check penalty by 2, making it 0 for light and heavy shields.
I think you will find that changing your tactics will help a little more because even on my fighter with a 50 AC I get clocked and beaten down. Attacking and tumbling out works well. Usually I stand across from the fighter and let him get a few swings in to get agro. Then if I pull it away I tumble around and through the party fighter and the creature runs into him trying to get to me and he can get agro back.
The rogue forum will have a lot more info.

[Sinth]

Cool thanks for the help everyone. I did some reading and decided im going to redo my enhancements and regear toward combat. Im currently lvl 10 and have sufficient skills to hit almost any trap I have encountered on a 2 or better. Ive decided to keep the shield, forget about the feat, and spend my next few levels on combat.

Dual classing is still out though, I like the idea of eventually having a lvl 20 pure. I can see though the immediate advantages of taking even 1 level as fighter, its tough not to after you do the math.

Thanks again for your help!