Had a quick play, a couple of minor suggestions.
1. The game speeds up but the tilt control doesn't which results in seeing something appear at the top of the screen but you never having the speed to avoid it. You could try adding a little acceleration. (vx += ax)
2. The score screen could do with a background that allows the scores to be clearly seen. A partially transparent panel would help.
3. How have you implemented the twitter functionality? I'm a little wary of entering passwords unless I know where they are going, have you considered using agon or openfeint or some indication of where the scores are stored e.g 'powered by cocos live'.
Having said that you've got further than I have, my first attempt has been sat on the shelf recently due to work commitments :D