I just thought I'd pass along my experiences, in case it can help others:
FWIW, I'm running a MacBook with OSX 10.5.6. I had a Keyspan USA-19 lying around from my old Palm Pilot. I tried to program my MiniPOV with this adapter and the Keyspan driver and it didn't work. I tried it with the PL2303 driver and that didn't work either. (I don't think the Keyspan has a PL2303 chipset, but I figured, what the hey?) Tried various delays, etc., still no go.
After reading an Amazon review where the reviewer said he was using a Trendnet TU-S9 for programing AVRs, I bought one for $13 at: http://www.amazon.com/TRENDnet-USB-to-Serial-Converter/dp/B0007T27H8. It seems to be working great with the PL2303 driver. (I also picked up a banned for my daughter's birthday to bring my total to $25 so I could qualify for free shipping.)
Keyspan's website says the USA-19 (with no letters after the "19") is designed specifically for syncing PDAs and they don't recommend it for general USB-serial uses. Keyspan makes several other variations 19h, 19hs, 19q, etc. Others say they have been successful with Keyspan adapters, so it seems that some work and some don't. But, at least in my case, the plain USA-19 doesn't work.
I hope someone finds this helpful!

