While I normally try not to participate in resurrected zombie threads, I'll throw two cents into the conversation here for posterity's sake...

I've used a weak oxalic acid bath on multiple frames, forks, and components. It's never damaged any paint or decals on my bikes and does a fantastic job removing rust. The only thing I've damaged using oxalic acid is the black oxide finish on a headset race. Further research after that incident confirms that oxalic acid isn't safe for black oxide coatings on steel, such as you'd find on allen wrenches.

I've also had good experience using Evapo-Rust and I regularly use the stuff for small parts, but it's significantly more expensive and may be cost-prohibitive for big items like frames & forks.