I don't know of anyone who has downgraded an X51, there was some talk about it. From what I understood, the x51 doesn't have a backup battery which could be a problem when it comes to removing the battery as you would lose all your data?
As for the TomTom not being compatible, they said they were going to release an update but haven't given a timescale. I know how you feel, I use my PDA for GPS so I downgraded but I have an x50v so it wasn't a problem.