Well maybe not by flashing ROM, but we can get anything on Milestone by SFB or Nandroid, INCLUDING 2.2, if someone was to manualy by ADB change their system files and replace them with 2.2 ones that is, a little harder for developers i gues than making custom ROM.
I tried doing one of the Nandroid 2.2 restores from a backup designed for the Droid: of course it didn't work. I just got prompted to re-flash the phone.
In part this is due to the fact that the current (stable and shipping, as opposed to unstable and hacked) bootloader will not boot unsigned kernels.
I also don't know whether Nandroid backups are CDMA-/GSM-specific; in other words if any of the telephony portions get stored in the Nandroid backup.
Also, the TELUS stock firmware SBF is now flashable thanks to the firmware decompressor/recompressor. I am no longer relying on a Nandroid restore on top of a Brazilian flash! I still do get occasional random reboots, though...still not sure whether this relates to the unlocked bootloader, the root I did, the overclock software, or some other factor. Maybe even the Nandroid restore. I don't think it's just the overclock, because I was having the same frequency of reboots before.