25 psi is fine for most situations. I am used to Jeeps where I can air down to 15-10-5 psi all day long, but remember the weight of the Hummer needs to factor in.
Don't go too low or you run the risk of your tire coming off, usually in most unopportune situations.
If you have to, or just want to go lower, invest in a set of beadlocks...
Travis
http://www.HummerTeam.com