the online slang dictionary says this:
(http://www.geocities.com/online_slang_dictionary/slang/p.html#top)
props n 1. praise, compliments. One usually "gives" props. Sometimes prefixed with "mad". ("I'll give Joe props for that crazy stunt.")

I guess this use was originally gleaned from the verbal meaning: to prop sth. up, i.e. to support, which of course, can also be used figuratively.