From the Ycombinator FAQ:
"Don't incorporate, though, if you can avoid it. Especially as an LLC. It's much more complicated for us to deal with existing paperwork than to start from scratch."
And as a personal note, I've found the same logistically - tinker together with friends on a project, see how you gel together, then worry about forming a company later.
There's also a contract that might help if one of you originally came up with the idea and recruited the other - you can create one that says basically that all the ideas and work belong to one person, the idea originator, but that they all transfer to whatever company you guys form if you actually get around to forming one.
I have zero legal experience, so take it for what it's worth, this isn't advice either way.