I just had the same problem, but the issue was not the theme, it was the TweetThis! plugin. The file was still on my site, it was just in the “wrong” directory. Moving it to where the error message said it was missing from (/public_html/wp-includes/) solved the problem completely.
Of course if I’d simply updated my plugins before I updated WordPress I never would have had this problem. Duh!
Cindy
<who believes in not going into panic mode when something catastrophic happens. At least not until you’ve Googled the problem :-)>