Wow, that email almost seems like they know what they're talking about.
I doubt the 700w/wx will be able to upgrade to WM6.
I have a Dell Axim x50v that had WM2003se and I upgraded it to WM5. Fast forward almost a year and I'm still on WM2003se on the thing. WM5 was awful on the x50v. So much so that Dell quickly stopped selling the upgrade.
Granted Palm and Dell are two different companies, I bet this would happen as well.
By the time the WM6 devices come out, it's time to get a new phone anyway.