This is not technically a bug, but since that's the only category you have for posting under i have to choose it.
There is a memory leak in LWJGL that causes the game to become slower and slower if you change dimensions a lot, for instance between Overworld and Nether (also all Mystcraft worlds).
The problem should be fixed in newer versions of LWJGL, so when they get the problems with OSX 10.6+ fixed the LWJGL shipped with Minecraft should be updated.
Linked issues
Comments 2
I find it disconcerting that a moderator simply resolves this issue and marks it as a duplicate without leaving a comment.
I have read through the other issue, and the two issues may not be related at all. I'm not saying they aren't. I just don't like the fact that a moderator resolved this issue without leaving a comment as to why they believe it to be a duplicate of the other issue.
I'm a professional programmer and have previously worked as a software-tester aswell, so i'm not totally lost when it comes to these things.
I know Grum is working with the LWJGL people on fixing the future version so Minecraft will work on OSX with Java 7.
Please do not select future versions, as you do not have them yet.