Well, he wouldn't ever publicly say he's vetoing it to protect Tor -- that would be political suicide, due to all of the CP thereon -- but he might still veto it for that reason, and claim another, cf. Iraq War.
Obama's not going to veto SOPA based on a (borderline) argument that it makes a tool which might be used by intelligence agencies, in some possible circumstances possibly illegal.
The argument that it needs lots of non-agency uses to hide traffic is plainly nonsense. If an intelligence agent (or subversive citizen) is using it in a country where it is banned they are going to be in trouble no matter if it is illegal in the US or not.
If it was a concern for US intelligence agencies (which I don't concede it is at all) then that can easily handle it themselves - they get an executive order allowing them to run exit nodes and then what they are doing is legal. Even by the borderline argument that Tor could be illegal under SOPA would only make it a problem for people running exit nodes - use by the general population. I doubt any intelligence agency would complain about being ordered to run a Tor exit node - I think they would love it.
I find it bizarre that people are even talking about this, when in the same source article where this was first raised it said the following:
"Browsers implementing DNSSEC will have to circumvent and bypass criminal blocking, and in the process, they will also circumvent and bypass SOPA orders." A successful injunction from the attorney general, Baker said, would shut down all shipments of a Web browser "until it's been revised to the satisfaction of his staff and their advisers in Hollywood."[1]
The US federal government provided funding to develop and deploy DNSSEC[2], too, and that is also useful for dissidents overseas. It also helps secure communications for billions (if not trillions) of dollars in ecommerce transaction. If that is ignored, do you really think Tor is going to get the bill vetoed?
It was a comparatively minor project from a minor agency where it is no longer developed.
Not only that, but Tor is too closely associated with the seedier sides of the internet for any elected politician to stand up for it specifically.
Thinking that the President will override a bill because of a minor thing like Tor is grasping at straws.