I'm still new to this whole root thing, but so far this is working for me:
1. I downloaded the 2.2.6 .apk version of the Market (I couldn't remember what the last version was that still had the old Market before 3.0.26 hit, so I settled on 2.2.6) - just Google it
2. I used Titanium Backup to Freeze the Market Updater (it's still at v1.0)
3. Used TB to insert the 2.2.6 .apk over the top of the existing 3.0.27
It's only been a couple of days, but normally the Market would update after the first time I used it - so far, so good. Now I'm looking for the final version just prior to 3.0.26 so that I can use that.
Good luck