Turbo mode is back for Linux as of the 2.6.33-rc5 kernel. w00t!

See this bug report:

http://bugzilla.kernel.org/show_bug.cgi?id=15064

Unfortunately the next Ubuntu version is going to be using 2.6.32. So you'll have to roll your own kernel on Ubuntu for awhile.

Here are some of my benchmark results running that kernel on a P55 i5 750:

http://imgur.com/7YVWo.png