I run Cyanogen on my Dream as well, but I got around the data blockage like so:
If you were quick enough to take advantage of the free HTC Magic upgrade they offered, you can just root the Magic and install CursorSense (at least 1.2.1). CursorSense is a mod of the official Rogers firmware, so as far as Rogers is concerned, you have a 'patched' phone, so they re-enable data access on your account. At that point you can switch back to your Dream with full 3G access.
I was without data for about a day (my Magic only arrived Monday, a day after they started blocking data), but since then I've been fine.
Rogers can take their mandatory upgrade and shove it. :p