Fedora 14 (up to date as of Jan 21 2011).
System info: Linux tv.localdomain 2.6.35.10-74.fc14.i686 #1 SMP Thu Dec 23 16:17:40 UTC 2010 i686 i686 i386 GNU/Linux
CPU: Core2 duo
Relevant HW: HVR 950 USB tv tuner and KWORLD UB-435 tv tuner (see lsusb.out and
             lsusb-v.out). HVR 950 has the correct firmware installed in
             /lib/firmware.
The problem(s) (see below) show up when using mythtv 0.24 (from rpmfusion).

1) There are bunch of i2c error messages appear right after boot when mythbackend
is started (see dmesg-right-after-boot.out). The mythfrontend is not able to 
show video from the HVR 950 USB device. If mythbackend is killed and restarted,
the two firmware loaded messages appear (see end of dmesg-right-after-boot.out)
and everything works correctly. I get video from both HVR 950 and KWORLD UB-435.
Both devices appear correctly under /dev/dvb/...

2) Now the strange thing is that if anything happens in any of the USB buses on the
computer, the HVR 950 will go out of whack again. Only 
reboot can cure this. For example, if I unplug one of the joysticks that is on a
different USB bus, this will happen. See dmesg2.out, which shows what happens
right after disconnecting another device and reconnecting it.

3) If I unplug the tuner USB devices and reconnect them, strange things will
happen. According to dmesg, the system discovers both of them again but I 
get only one dvb device under /dev/dvb. To get things going again, reboot is
required.

Note that these problems (IMHO) are related to the kernel drivers and not mythtv.