indeed, he poured joffrey the whine, but he didnt put the wine in the bottle.
Tyrion would would be way more clever then that.
naturally Cersei think its him.. even if she didnt think so she still would accuse him since she has been looking for a reason to get rid of him for seasons!
on the sidenote what ive always wondered is: why did Tywin name the imp after himself? (more or les, or his father perhaps) and not jaime?
would make alot more sense to me if Tyrion was named Jaime, and Jaime Tyrion