This report (which I found via this site) tells us more. It's no surprise that tweets reflect the regional, social, ethnic and age variations of the tweeter, but it's intriguing that such variations can be mapped so closely. For example, to find that intensifiers like hella, meaning really/very (as in hella tired) or dead ass (as in dead ass tired), or even a comparative version like AF (as in tired as f**k) can be traced to specific cities or states is quite a neat idea.
So, as we pointed out back here, Twitter is actually working to preserve some regional variation rather than lead to the dialect levelling that so many people predicted.
edited on 13.01.11 to add:
There's another blog post about Twitter that fits in with this recent research here.
edited later on 13.01.11 to add:For an LA blog's take on their Twitter slang/dialect try here.