Alright, I'm currently thinking about registering my game with Steam's new Greenlight program. Only problem is I don't have a trademarked title yet and I read the government's registration process can take a little while. (and  I don't have at the moment) So naturally, this got me wondering if it is a sound idea to proceed without one.

So my question is are there any serious pitfalls I should worry about if I start advertising without a trademarked title? (Assuming it doesn't infringe upon anyone else's property of course)

Depending on your jurisdiction, simply using your title (and perhaps marking it with a ™ symbol) may qualify it for protection as an unregistered trade mark.

The Wikipedia article is pretty short, so let me quote it in its entirety:

An unregistered trade mark is a trademark which does not benefit from the protection afforded to trade marks through registration.

They may however benefit from protection due to other features of the law in relation to trademarks, such as protection for unregistered marks in the United Kingdom resulting from Passing off law.

In the United States, neither federal nor state registration is required to obtain rights in a trademark. An unregistered mark may still receive common law trademark rights. Those rights, for example, may extend to its area of influence—usually delineated by geography. As such, multiple parties may simultaneously use a mark throughout the country or even state. An unregistered mark may also be protected under the federal "Lanham Act" (15 USC § 1125) prohibition against commercial misrepresentation of source or origins of goods. Unregistered marks are also protectable in the United States under Lanham Act §43(a).

(Authors: Wikipedia users 195.157.197.108, Mmmbeer and others; used under the CC-By-SA 3.0 license.)

Ps. The main trademark article on Wikipedia is significantly more detailed, and well worth a read.

Well, I do know that you get copyright just for coming up with it, so if you put proof (on your website or something), then you shouldn't run into too many issues...

Then again, I'm not a lawyer, so don't trust my advice too much!