I'd add that it's worth noting the difference between "licence" and "licence agreement". Most software has a licence, though that doesn't mean the user has to agree to it; a licence isn't necessarily the same thing as a contract (indeed the GPL makes the point that it isn't a contract, and no one has to agree to it).
Well, yes and no. As soon as the user tears the shrinkwrap open (on a physical copy) or installs/uses the program, there is an implicit (and sometimes explicit, by clicking the "Yes, I agree" checkbox) agreement.
The user may not respect the agreement or all of its terms, and he may not even have read it (most people don't read!) but as far as the lawyer cares, it's certainly an "agreement".
It is not possible to legally use the software otherwise either -- usage of a software is only permitted with a valid license, and a license (usually, except for the Do-WTF-You-Want license) contains a paragraph that states the license is only valid if you agree with the terms. The user can of course still use the software illegitimately, but in that case it will be rather hard to press charges against the author.
Though, in the USA, maybe even that is possible. I've heard of a burglar who cut his arm entering a house and sued the house owners for having glass windows that can cut your arm when you break through them. Not sure whether that was a hoax or real...