I've asked TomTom whether Navigator 5 is compatible with the X51 and had this reply:
Thank you for your mail and and sorry for the late response.
Unfortunately, the Dell Axim X51v is not compatible with the navigator 5. But, we are currently testing other devices to see if they are or will be compatible. Please keep an eye on our website.
With best regards,
The TomTom Customer Support Team
As the X50 IS compatible, it's presumably the Mobile 5 that's not compatible with Navigator 5. I've asked whether they'll be updating Nav 5 to work or not.
Chris