Years and years ago I think we learned at school that the comma was compulsory before TOO, but that certainly doesn't seem to be the case in real life.
The two examples suggested by Patrick obviously mean two different things, but I'm not sure it's the comma that makes the difference.
The Chicago Manual of Style Q & A column (of which I am inordinately fond) agrees that asking a comma to make a difference of meaning in a sentence like the one discussed is asking too much of a comma. (Their example is different, but the issue raised is the same.)
Use commas with too only when you want to emphasize an abrupt change of thought:
He didn’t know at first what hit him, but then, too, he hadn’t ever walked in a field strewn with garden rakes.
In most other cases, commas with this short adverb are unnecessary (an exception being sentences that begin with too—in the sense of also—a construction some writers would avoid as being too awkward).