Have you tried WiFi + Power + Reset? Obviously, if you really messed up the bootloader that won't work, but it's worth trying.

The only known solution otherwise is to do a reflash by wiring a cable to the JTAG pins on the x50 motherboard, however I don't think anybody has worked out what the pin arrangement is yet, so you can't unfortunately.

If it's still under warranty, I guess you could try returning it to Dell. You'd have to find out whether flashing the wrong bootloader is covered by the warranty though...

Also PocketPCTechs might be able to repair it for you, you could contact them and ask.