Suppose that a law required you to give up certain constitutional rights, such as privacy, in order obtain government benefits to which you were entitled. Would that be constitutional?
Not according to the doctrine of unconstitutional conditions.
I'd note however, that in criminal cases involving weapons and contraband, it is a frequent condition of probation that the defendant agree to give up his Fourth Amendment rights against unreasonable search. He must allow a peace officer to search his person, residence, and automobile in the day or night, without a warrant, with or without probable cause. This invitation to suspicionless rousting of known probationers who don't seem to be doing anything wrong is currently on appeal, I believe.
Doesn't this violate the unconstitutional conditions doctrine? On its face it so seems, but you must remember that virtually all personal constitutional rights may be WAIVED, provided done so in a knowing, voluntary, and intelligent manner. This is the rubric recited by judges when taking pleas of guilty, which involve waiving a host of trial rights. Are there any rights that cannot be waived? How about the jurisdiction of the court. May a defendant in traffic court plead guilty to capital murder? I don't think so. This is the only right that a defendant may not waive, so far as I know, and there may be some question about that, depending on how you define jurisdiction. For example, does a court lose jurisdiction to convict and sentence a defendant after the expiration of the statute of limitations? Doesn't seem right, does it. But suppose the defendant WANTS to plead guilty to a lesser, time-barred offense as part of a plea bargain that gets rid of a current, heavier offense. May he do so? May the court? May the defendant waive the statute of limitations? You can look it up, as it's been decided in California.
Here's an article from National Review Online by Prof. Matt Franck on the reference by new Chief Justice Roberts to the doctrine of unconstitutional conditions in the FAIR v. Rumsfeld decision announced yesterday.