I did read your thread, but didn't try that approach. Since the cover art in iTunes was accurate, it seemed to me the error had to be in how the database in the iPhone connected song to art. Pasting a correct photo back onto itself in iTunes seemed unlikely to help. I'll definitely keep it in mind if it happens again and I can't resolve it another way -- sometimes the thing that doesn't seem like it should matter ends up being the solution, right? Thanks for the suggestion, much appreciated.
What I tried next was just restoring the iPhone -- which I hadn't done yet, had only synched it. That seems to have done the trick. I checked about 20 songs that I'd noted as mismatched previously and they were all correct. Haven't checked too many more songs yet, but it looks as though it's repaired.
Restoring is a fairly easy solution, but it does take some time and is annoying if you're planning to head out and expect to use the iPod part of the phone. Hope this mismatch doesn't happen too often!!