When politicians/advertisers/whatever send mass emails it can take a long time to cycle through all the master list. Sending an email to thousands of supporters can take hours. I don't know how Obama's text database will work, but some people will probably be getting their text before others just because it takes a while to get through the list.
I think there's the potential for a glitches, too. Some people may not even get it until the next day, for example. Could be embarrassing for the O-man.
Just some thoughts, while I keep waiting for that text. Maybe a commenter who knows more about this could offer some insight in the comments.
P.S. Editor Jon totally fooled me earlier today by sending me a text from an unknown number saying Obama had selected Joe Biden and they would be appearing together on MSNBC at 7pm this evening. Slick, Jon. Slick.