After reading Techcrunch's The Top 40 iPhone Apps of 2010 and finding out that the "Find My iPhone" app is now free (previously required a paying MobileMe account), I tried to get it up and running, but ran into some issues. Here's how I got it working.
Apple provides a walkthrough/tutorial of setting up the free-feature via the following article: Apple - iPhone - How to set up Find My iPhone
The problem I ran into was a weird "bug" that said that the Apple ID I was registering with wasn't a valid account (or rather, my username/password combo weren't valid). After creating some fake/dummy accounts and successfully registering for it, I realized that the issue was that my MobileMe account was just a username/display name, and not an email address. This seems to throw Apple off it's game. The resolution is simple.
Manage Apple Account is an area that allows you to change your name, location etc. Login there with your non-email-based Apple ID. Change your Apple ID to a full, valid email address, save.
Then, go back to your iPhone and try registering your MobileMe account via Apples Apple - iPhone - How to set up Find My iPhone instructions.
This worked for me. Hopefully this helps someone else.