HP 7350 work-around
After upgrading to Leopard on a PowerPC-based iMac, I too could not print with my Photosmart 7350. I got the "GenericClass: waiting for device..." error. I've read that the Photosmart 7350 is not supported anymore for Intel-based Macs, but I haven't been able to find any info as to whether PowerPC-based Macs suffer from the same problem.
Therefore, in this information vacuum, I decided to press on, and I've found a crude work-around that works for me. Maybe someone can use this info to figure out how to fix this problem. Anyway, this is how I get the printer to print. I put the computer to sleep. Just as the screen goes dark, I immediately hit the space bar, waking the computer up again. This somehow reestablishes a connection between the computer and the printer, allowing me to print. It will remain "fixed" until the computer sleeps again.
I don't know if this will work for anyone else, but i can say that using this method, I can print to a 7350 while running Leopard.