First let me say that I disagree the Spurs will pay over $4M and over 3yrs to keep Oberto...but then again, neither you or I really know what they'll do.
And if someone throws $20M at him, whether it's the Mavs or not, I think he'd find it hard NOT to sign that offer sheet and leave it in the hands of the Spurs. That way, he can always say that the Spurs had the opportunity to match, but didn't...and place the onus on them.
About the specific teams...
Bulls do need a low-post threat, but they also need post players in general (Sweetney and Brown are FAs). Skiles and the GM are also fond of "character" and "team" guys. I'm just saying it's possible.
Accidentally threw the Grizz in there because of the report two summers ago that West liked Oberto...but West is gone from Memphis...my bad.
Raptors have one legitimate center (Nesterovic) and an affinity for international players. Colangelo was willing to take on Nesterovic's big contract, so what makes you think he won't overpay for a role guy like Oberto?
And there are probably a few other teams that will enter the bidding that we're not considering...