I've done 3 Wherigo caches in my area and played the Wherigo Tutorial cartridge without any issues with the nüvi 550 (Software Version 3.40, Wherigo Engine 2.22).
While building my own Wherigo, I found that my cartridge worked flawlessly on the Wherigo Builder's Pocket PC emulator but kept crashing in the field on the nüvi 550 (problems mentioned above; locks up or switches off). I've tracked down the problem to the "Show detail screen zobject" script Action (supposed to force a certain screen to be shown to the player, such as a particular Task or Location). When I removed all of these Actions, everything now works without a hitch.
(When I download cartridge files from Wherigo.com, I get the ones for "Garmin Nuvi 500" in the drop-down box; when I build my own using the Wherigo Builder, I compile for "Any Device".)
Hope this helps.
