I got this figured out. What's puzzling is that it worked for you guys.
If I pasted the Anandtech Hot Deals Forum URL (
http://forums.anandtech.com/rss.aspx?forumid=1&catid=40) into the readRSS.php page directly, then echo'd the rssURL parm, it cut off at "forumid=1". $_GET saw &catid=40 to be a totally separate parm.
I got around it by doing a foreach and ignoring the ones I didn't want, concatenating all of the ones I wanted to keep into one var. I then used that var in readRSS.php instead of $_GET[rssURL].