Mac going to sleep when lid opens, waking up when lid closes

The title prety much says it all.

If I manually put it to sleep, I can then close the lid and it will keep sleeping, and will wake up when I reopen the lid.

But if I then just close the lid, nothing happens. I can see the screen turns off (apple gets dark) and the power light turns and stays on. If I reopen the lid, the screen briefly turns on, then turns back off, and power light starts blinking. The computer wakes up if I close the lid again.

Using Lion, but it didn't do that before.