I actually work for FCD and can help anyone out who might need/want tickets when they go on sale. (It isn't official yet, but reading tea leaves around the office, it looks like a good bet.)
My direct line is 214 777 0225. My momma calls me Kevin.

You can also email me at klindstrom@fcdallas.net.
The best way to make sure you get tickets if you're in the DFW MetroPlex or willing to drive to Dallas for FCD games is to have season tickets - you get in on the pre-sale ahead of anyone else (even ussoccer, I believe). If you don't already have season tickets, I can sign you up at the same time and get you in the pre-sale.
For those who have a problem with the opponent being Guatemala, a few notes. One - in ten years at the Cotton Bowl, there were few if any significant problems for either Dallas Burn games or the US game. Event staff does a good job of making sure there aren't any problems - including the Chivas matches that have been held.
Two - FC Dallas has an interest in it being Guatemala as they have a certain player on their team who might appreciate the game. If you're not familiar with either Guatemala or FCD, Carlos Ruiz is arguably the best forward in CONCACAF and in MLS.
Cheers,
Kevin