While upgrading from wordpress 2.x to 3.0 latest release there could be error saying:
call to undefined function is_multisite
This error can be due to incomplete upgrade or imcomplete file transfer while upgrading from wordpress 2.x to 3.0 automatically.
What’s the solution? here is is:
If you are getting error: call to undefined function is_multisite after wordpress 3.0 upgrade then simply do any 2 of these steps:
1. do a manual upgrade of the wordpress (detail instructions here)
2. Hire a professional (I am willing to volunteer to be hired :D )
If you are not technically oriented person then i think the second option is best for you.