I've heard the collecting bananas increases your speed, is this true? As I don't seem to speed up when I get a bunch. Although that may be because the speed of each one is too subtle to notice.
They do marginally increase your speed, but I have never noticed it being that much of an increase. Also, the increase stops at ten bananas - you can theoretically collect more than that, but they won't help you.
As Toomai pointed out in comments, you can use the VITAMINB cheat, which allows you to collect an unlimited amount of bananas.
Collecting Bananas increases your speed by a small amount. It's unclear whether or not each single banana increases your speed slightly, or if it increases every 2, 3 bananas, etc. You'll stop speeding up after you grab 10, but having extra is good insurance against attacks.
There are also two cheat codes that relate to bananas:
VITAMINB: There is no longer a speed bonus cap of 10 bananas
BOGUSBANANAS: Collected bananas cause you to slow down rather than speed up.
As far as I can know, these two cheats are stackable.