How did you go flashing a different ROM Roostlick? Which ROM did you try (URL?).
I rooted my Australian Vodafone Magic the day i got it using the same instructions st0kes mentioned. I didn't replace the 'su' program since i didnt need it and seemed like an unnecessary security risk.
Data usage was fine (normal) for the first 2 weeks. Then it started downloading 4.5mb chunks every 2 hours.
Got close to my 1gig cap, so i turned off all data synchronisation. I have just received my bill today to find the cap was blown.... and that these 4.5mb downloads have continued.
Obviously looking to fix it, since i'm already 400mb through this months cap!!
edit: looked at Settings>About phone>System updates> and i can see "Android Update 1.5" waiting to be installed... no surprises its 4.2mb. I hit "download' on the update and it fails silently after downloading, giving a download prompt again.
Found some more info on the OTA updates in JF's blog here
I'm currently testing the chmod solution:
chmod 000 /cache/download*