You can always try this guys method, with the reinstall HP drivers edited out...
Of all the things I've done, I'm convinced that what fixed it was resetting the printing system. A forum linked to from another place I posted this question mention that it helped others. I found how to reset the printing system in Leopard by searching for it online.
For good measure, did the following:
(removed)
* I went to System Preferences > Print & Fax and right clicked on the left had sidebar. The contextual menu had the command "Reset Printing System". This warned me that it would delete all my printers. I said OK. Sure enough all my printers were gone.
* I re-set up the HP 6210 while connected directly through USB using the HP setup Assistant. This process adds the printer both as a printer and as a Fax in System Preferences > Print & Fax.
* I opened the print queue and the utility and print a test page using this direct method.
* I disconnected the HP 6210 from the computer and connected it to the Airport Express.
* I went back to System Preferences > Print & Fax and clicked on the plus sign to add another printer
* (additional note) In the resulting dialog box, I tried adding the printer as an IP printer (using the Airport Express' IP address) but for some reason could not find the right driver (I was looking for one with the term "OfficeJet"--more on that below). I picked a different driver knowingly and got a weird message about the software being incomplete. Sure enough, printing to this new printer added this way with the wrong driver did not work at all. I decided to try the OS's own automatic way to add the printer (see below).
* In that same dialog window referenced above, I clicked on the leftmost icon (I think it says "Default") and the (at this point wireless) printer was already on the list as a BonJour printer. I made sure to give it a different name than the already added, direct USB connected (same) printer by adding the word "wireless" so in the printer list and print dialing windows and it's print queue it's called "HP OfficeJet 6200 Series wireless" as opposed to "HP OfficeJet 6200 Series" which is the original direct connected (same) printer. And the driver the OS picked automatically is "HP All In One 5.3.1" (as far I can remember--it could be something a little different).
Likely bonjour will fail to find it, HP printers and wireless seem to be giving people trouble.
