If MythTV, after an otherwise innocuous kernel upgrade complete with an upgrade of the required ivtv driver, fails to show live tv even though ivtv itself works, check the settings and make sure the second tuner on the pvr500 card doesn't point to nowhere. Still not sure how a kernel upgrade could affect the setting inside the MySQL db, but what can you say.
On the other hand, the upgrade of MythTV itself from 0.19 to 0.20 was seamless. Oh well.