Summary: Does the creature with Unleash have a +1/+1 counter on it? If so, then it can't block. That simple.
The first thing one should do when wondering how an keyword ability works is to look up the abilities it stand for. Unfortunately, the exact rule text of the ability has not been released yet, and no official answer has been given. That means that any answer is going to be opinion.
The reminder text ("It can't block as long as it has a +1/+1 counter on it") indicates it doesn't matter how it got the +1/+1 counter. (Reminder text is not always 100% accurate.)
The official Wizards blog says it doesn't matter how it got the +1/+1 counter. (They have been wrong before.)
Everyone on the Rules Q&A forum believes it doesn't matter how it got the +1/+1 counter.
All indications point to Unleash being a straightforward static ability that optionally gives a +1/+1 counter, plus a straightforward unrelated static ability that simply checks for +1/+1 counters. If so, it doesn't matter how it got the +1/+1 counter.
That said, the rules have now been released, and the earlier answers have been deemed correct.
702.96a Unleash is a keyword that represents two static abilities. “Unleash” means “You may have this permanent enter the battlefield with an additional +1/+1 counter on it” and “This permanent can’t block as long as it has a +1/+1 counter on it.”
Unleash is static ability that optionally gives a +1/+1 counter, plus an unrelated static ability that simply checks for +1/+1 counters. It doesn't matter how it got the +1/+1 counter.
(Put a +1/+1 counter on target creature. Put a +1/+1 counter on target creature.)a form of control. If your opponent chose not to put unleash on that creature, then clearly they did so because they intended to block with it (otherwise, there's no reason not to). You interrupt their plans with it. Now, it might not seem like a good idea to buff your opponent's creatures, but that last block might be the difference between you winning or them winning. It may take a turn off their clock that they weren't expecting. – corsiKa Sep 21 '12 at 17:37