jtzell - thanks for the suggestions!!
For filter by category: you can do that right now by typing the category prefix for what you're interested in. For instance, if you want to filter for LARP events, search for "LRP" and it'll come up with all the larp events. RPG for role playing games, etc.
I know there isn't a nice drop-down for it, but in the end I decided that it took up too much room on the screen and removed it. There may come a point where I implement a "swipe from the side for advanced search options" but that is truly a mess and I'd like to avoid it. Screen real-estate on large phones or tablets isn't a problem, but on small phones it's difficult to fit it all in.
Regarding the search problems - I actually have fixed that as of about ~20 minutes ago.. It should both fix the problems in the first place (it shouldn't get "stuck" anymore) and it should also allow you to click the "X" button to clear everything and reset to all events in one tap.
I have submitted updates to both the Android and iPhone, but it'll be a while before they come through the pipe (probably tomorrow morning for Android and ~Tuesday evening~ish for iPhone. If you're on iPhone, look for the 1.5.1 update in a few days, and on Android it should be the 3.5.1 update by tomorrow.