I suspect whether a UPS would be helpful would depend a lot on the UPS.  Some provide fairly good power-line conditioning while running off the mains and some don't do anything but bridge the line through unfiltered as long as there is usable external power.  Also, bear in mind that the vast majority of UPSs will introduce a small glitch when transitioning to battery and back again to the mains.  These glitches typically last only a few ms, and can't be helped because the UPS needs some time to determine that the line power has gone off-normal and respond.  It then must fire up its own circuitry, often meaning there are several cycles of substandard power before things settle down.  And most inexpensive UPSs do not produce anything close to a sinusoidal waveform.  It is presumed that most computer power supplies have plenty of internal capacity to absorb all these things in stride (and most do).  If your plug's power supply is particularly sensitive, though, any of these might be enough to cause the corruption you'd like to avoid.

Question: what is your voltage?  I'm wondering if perhaps it is operating at the lower end of the plug's limit.  Do you have access to a variac?  If so, and if your voltage is low, you might try increasing the voltage by, say, 10% and seeing if that has an effect on things.

Good luck!

Just out of curiosity, why did you recently move the forum from openplug.org to plugcomputer.org?  I noticed a few days ago that Firefox stopped remembering my password and I had to re-enter it again.  I had to change some hard links in my bookmarks to get things working again.  (For example, I find keeping a pointer to http://plugcomputer.org/plugforum/index.php?action=unread on my bookmarks toolbar handy for finding and reading the new posts here.)

Also, what is the intended use of the openplug.org vs. plugcomputer.org domains going forward?  There seems to be a lot of overlap there, although right now they are both distinctly different websites.  Just curious.

I have the same idea in mind (HDHomerun + the Sheeva Plug for saving the streaming DTV).  I have not received my Plug yet, so have not had a chance to experiment with it at all.  However, on paper, it looks like a reasonable match.  Basically, the Plug would be doing little real-time processing in this application -- it's simply serving as an I/O engine.  I normally get about 25MB (Megabytes) per second thruput when I dd to a USB hard drive on an Intel-based PC, so I doubt disk bandwidth will be an issue.  I question whether the Sheeva is capable of sustaining anywhere near its Gb ethernet NIC's thruput, but I expect it to be able to handle several channels worth of 20Mb (Megabit) data just fine, even if it's used on a 100BaseT Ethernet network.  (My personal experience is that the HDHomerun box does just fine feeding a PC thru a local switch, but my 100BaseT network tends to drop packets when feeding a PC at the other end of the house through three switches.)

One final comment: you may want to consider which type of file system you employ for saving your programs -- I understand some have better characteristics than others for large-file work.  Thus, say, deleting a previously recorded program while another is streaming may be problematic to the real-time stream.  (See, http://www.mythtv.org/docs/mythtv-HOWTO-3.html )