I just registered because I was having this same issue (and yes, it is the feedback from reflecting the speaker directly back into the mic with a very close but not covering hand, etc.).
After thinking about it, I tried intentionally activating the mic, then deactivating it, using the most minimal process available - the Voice Recorder! Start it, start recording, stop recording (I paused it, myself) which stops the mic, cancel the recording and voila! The mic is now off (and hopefully the speaker, too)! So far, phonecalls (even on speaker phone) don't mess it up, BUT restarting the phone will require you to do this again.
PS: I can't help but think how much power this glitch uses, and if we're all being bugged (spied-on) by this. Hmm...