One of the things that we've done in Whidbey is add some extra IL instructions to improve the debugging experience. This allows you to (for example) set a break on a closing brace.
Because /o- is the default setting, this means that performance if you just compile with “csc” will be slightly degraded in Whidbey. If you've been building without setting /o+ and you care about perf, you will want to change your code to throw /o+ explicitly.
If you're using VS release and debug configurations, this will be set automatically for you.