/dev/ttya is probably not tied to the USB port. I have a handpring visor that connects via a USB cradle, and it shows up on /dev/ttyUSB0 or /dev/ttyUSB1.
Interesting. I get the dialog in my pilot-manager but my TREO tells me that there was no connection (warn message). The log of my TREO tells me everything went fine. The status bar does not move at all.