What if the telecom companies provided assistance, but the assistance was NOT in accordance with any law passed by Congress, but instead was against a duly-passed federal law?
That's the legal/factual posture created by the combination of the Terrorist Surveillance Program, as described by the administration, and the federal surveillance statutes at 50 USC 1801 et seq / 18 USC 2511.
Now, as a practical matter, the issue is considerably more complex than just that. For one thing, an aggrieved party (as described in 50 USC 1810, the section of law that give plaintiffs a right to sue the government) would have to make a showing he or she was in fact surveilled. I think this hurdle cuts off virtually all suits.
If that doesn't cut off a suit, the government can assert state secret. It did so successfully with the al Haramin case, where the plaintiff had mistakenly been given evidence that it had in fact been surveilled. The plaintiff was denied use of this evidence by a decision of the Ninth Circuit (al Haramin case).
Finally, as a matter of general principle, it's possible (IMO), for a president to conduct an individual surveillance that is outside of statutory boundaries, but inside constitutional boundaries. Clinton did so with a warrantless physical invasion of Aldrich Ames' house, for the purpose of setting bugs.
Whether or not the specific actions undertaken by the TSP (which are in contravention to US federal statute) are constitutional can't be analyzed without resort to facts, and those facts aren't (and likely won't be) made public.
Rather than analyze all the reasons why a suit would probably fail, how about we pass the immunity, get the surveillance, and save the legal analysis for elsewhere.