Well, I just moved my betta into his own tank (because I saw him nip one of the frogs) and the new tank (a 2.5) has a little mini filter in it, a whisper mini.
I had to leave for the weekend, so as I was walking out the door I checked on him one more time - and it looked like he was stuck to the filter intake!!
Of course, I completely freaked out and turned off the filter, but he didn't move until I poked him, when he drifted to the bottom and sat there looking terrified. I still had to leave, I've had my roommate watching him and she says he's doing fine. I left the filter off.
Why on earth would he get stuck to the filter intake?! Is it okay to have a filter in a 2.5?! I thought it was, but maybe I missed something....