This really has me baffled...
I saw this error pop up in this thread as well..
But don't seem to find the solution.
I'm running buddypress 1.0.3 and wpmu 2.8.4 and get this as soon as I register a new blog...
[function.array-merge]: Argument #1 is not an array in /home/thatmlmb/public_html/wp-content/plugins/buddypress/bp-xprofile/bp-xprofile-signup.php on line 261
The code on line 261 is
$bp_blog_signup_meta = array_merge( $bp_meta, $blog_meta );