I've seen some immensely tacky steampunked stuff, but so long as it's kept simple I really love the look of heavy "industrial"-looking relics.
There's some ridiculous stuff on this page
Anybody done any heavy relic-ing? How'd you go about it?
That's pretty cool. I actually help run a Steam Punk Club/Cabaret and have helped before with sets, costumes and do all the media design for it and while I haven't applied it to guitars, most of the stuff can be transferred over easily enough.
Usually to make woods appear more 'utilitarian' and used, I tended to go to bare wood and use watered down brown washes and do a few coats to bring the wood up to a more vintage aesthetic. For metal, I usually like to attack stuff with a blow torch but have taken a few small metal items to be brass plated down at the local electro plater which would work for bridges and other metal parts that have no greased moving parts.
As long as it isn't adorned with a stack of cogs for no reason it's all good. :P