@dixon_jones, a well known and respected SEO, asked Google's JohnMu in a Google Webmaster Help thread why Google shows the old Twitter URL versus the new Twitter URL in the search results.
For example, my old Twitter URL is twitter.com/rustybrick but the new style for the Twitter URL is twitter.com/#!/rustybrick.
Yes, Twitter uses the new AJAX schema for the URLs. So why does Google show the old version without the #! in the URL?
JohnMu explained because Twitter redirects the #! to the old URL.
Google crawls AJAX URLs using the AJAX-crawling scheme which results in the URL looking like this instead http://twitter.com/?_escaped_fragment_=/rustybrick. When you look at what Twitter does when you access that URL, bots are redirects to the old URL.
#1 Server Response: http://twitter.com/?_escaped_fragment_=/rustybrick
HTTP/1.1 301 Moved Permanently
Date: Wed, 22 Jun 2011 12:46:47 GMT
Server: hi
Status: 301 Moved Permanently
X-Transaction: 1308746807-27933-10397
X-Frame-Options: SAMEORIGIN
Last-Modified: Wed, 22 Jun 2011 12:46:47 GMT
Location: http://twitter.com/rustybrick
Forum discussion at Google Webmaster Help.

Comments:
Joe Youngblood
06/22/2011 06:39 pm
i like it, ajax urls suck
Jonathon Hewitt
06/22/2011 06:50 pm
All you have to do is sign out of twitter and you'll see what a search engine see's. After-all a search engine or bot may have a hard time logging into Twitter. Googlebot or not, no bot is that good yet.
Markus Jalmerot
06/22/2011 07:40 pm
I've been noticing that google index the Twitter version without #!/, but didn't know why. Glad you guys found out the reasons behind ;)
Punishat Lehry
06/22/2011 07:50 pm
Appreciate it for helping out, excellent information.
Web Element
06/23/2011 08:27 am
Always wondering why this happens. Thanks for the information.
Ryan Erwin
06/27/2011 06:02 pm
Agreeeeeed. I wasn't sure why this was. I know we use the most current code on our site, www.ctaxrelief.com and we want to make sure we are using the most up to date code.
exinco
08/30/2011 05:51 am
i'm glad if i can switch back to old version of twitter. i can't verify for some program that i'ved join some times ago
exinco
08/30/2011 05:52 am
i'm glad if i can switch to old version. i cannot verified some program i was join some times ago with new version of twitter