It is sometimes quite common for this to happen when Google perform a rollback, I see it many times with new domains I get indexed, but eventually they appear in the SERPs. My latest domain appeared then dissapeared within days, then within a week it was listed again.
It is quite possible to have been sandboxed, although I cant see this happening if you have performed only a few bookmarks, a few hundred is a different story. You have to be careful when indexing new domains to avoid the sandbox, it could be back within a month or two, yet it could take half a year for it to return, maybe even not at all, there is no guessing.
I guess one way to find out is the regularity of Googlebot spidering your sites, my domains get spidered daily, I think usually sandboxed sites dont get spidered as frequently, maybe once per week/2 weeks? Things could have changed as it has been a while since I got a domain sandboxed, so if somebody could clarify? If you have AWStats then that is a great way of checking for spider activity.
When I buy a new domain I usually submit 1 digg bookmark to get the domain indexed, then I let it mature for a month in Google before performing any SEO campaign, thus in the meantime I can work on my other sites SEO.
Either way, I hope your domain returns soon enough, it is frustrating to put the research into buying a good domain name and have it sandboxed.
You can find more info at wiki:
hxxp://en.wikipedia.0rg/wiki/Sandbox_Effect