It's not open to the public, but the largest bat colony in the world, Bracken Cave, is located between Austin and San Antonio:
http://www.batcon.org/discover/bracken.html
I have a friend who works for the Bat Conservation Org. and was allowed to visit the cave about a year ago. Here's a few shots I took while there:
http://www.pbase.com/wedeking/bracken_bat_cave
I must say that both the cave and Congress St. bridge emergences are spectacular sights. I haven't seen the Round Rock bats yet, but I expect it too is a sight to see.
I did find the photo shooting conditions quite challenging. Large dynamic range difference. Fast moving objects at dusk. And at the time, a camera which didn't have a very good high ISO capability. I would love to go back someday and give it another go.
This shot is a combo of two exposures, trying to deal with the large dynamic range. (Admitedly, not that good of a photoshopping job) :D
Greg
http://www.pbase.com/wedeking