since you're trying to incorporate steampunk style into a child's space, i would HIGHLY recommend watching the following films for inspiration: hayao miyazaki's "castle and the sky" and "howl's moving castle" and also katsuhiro otomo's "steamboy". i think they'll show a more whimsical, playful side of the style.
i like the idea of the large gear mural on the wall. simply a large silhouette of a few gears & cogs in a color a shade or two different from the wall color or simply with varnish so it's shiny will give that subtle look.
decorate mostly with greys & browns with small touches of color for accent.
galvanized bins and wire baskets for storage & organization.
old metal tonka trucks on the floor and model airplanes hanging from the ceiling.
ooh. this is fun. such a different take on a nursery than i'm used to seeing on this site. i'll post more ideas if i think of them...

You could put some gears and mechanical parts on a canvas -- and then spray paint around them in various colors creating a sort of silhouette effect. I've seen it done and it looked pretty cool. The spray paint was in 'clouds' of different colors and not one thickness. The end product was sort of abstract expressionism meets machinery.

If you are looking for a large-scale subtle wall mural of gears I think it would be interesting to paint the wall a mechanical shade(light gray maybe?), and then constrast the gears in the same shade, but in a gloss paint. Subtle, yet stylish.

http://thesteampunkhome.blogspot.com/search/label/kids

this site has some interesting ways on interpreting the steam punk style into every day life. the link provides a list of posts specifically about ideas for kids, but many of the site's posts are interesting and useful.

This fabric line has cogs on it? Not sure if this helps/is what you are looking for??

http://www.shopcraftyplanet.com/store/product.php?productid=130540&cat=8&page=1

Karen -if you would be interested in a set of old helicopter tail blades (not Functional!) contact me.

Not sure if this would fit in with the Steampunk theme, but I thought this print was awesome.

http://www.etsy.com/view_listing.php?ref=vt_related_1&listing_id=28865794

the first thing i thought of was this room at the ace hotel in portland: http://www.acehotel.com/portland/rooms

(the one with the grey wall and the large line drawing of gears/machinery in white)

you might be able to replicate something like this on a small scale by finding old schematics in books and making collages out of them.

Thanks for all the great suggestions! I'm looking forward to tackling this project, and you guys have provided some wonderful resource ideas.

Home decorating are always been a good point to every house owners. The area rugs at naturalhomerugs can be of big help also. Please try to visit this to reinvent the decor you want.

maybe you could use these somehow?

just came across a cog/nut home accessory at cb2
(it's on sale, so look soon, $11 each)

since they are made of iron, maybe you could use them
as bookends on a higher shelf (so baby couldn't reach and hurt themselves), or a doorstop? or just mount on the wall or place on a high shelf for decoration. maybe you can think of something more creative though...

oops, sorry, forgot to include the link!

http://www.cb2.com/family.aspx?c=910&f=5129

If you search etsy.com for "steampunk decor" it gives you some really cool options -- artwork, wall decals, pillows -- but my favorite was this lamp made from a vegetable steamer (pun intended?)

this is probably way too expensive for a kids room, but would be perfect!!
http://www.overstock.com/Home-Garden/Clock-and-Gears-Wall-Art/2403972/product.html
(clocks and gears wall art @ overstock.com)

I love steam punk! it can be done with out being too themey too.

love the cog/nut accessories from cb2 and also love the suggestions of using the ace hotel/portland as an inspiration source... i stayed there a few years ago and it definitely has a modern yet vintage/classic feel to it that is really stunning.

ooh, and can i suggest a steamer trunk as a toy box?