I have this problem as well. I installed wordpress 3.1 on my 1and1.com host and am trying to add BP 1.2.8.
(uiserver) > uname -mrsn
Linux infong576 2.6.28.8-20100125a-iscsi-ntacker-fasync-mremap-grsec i686
From my debug.log:
17-Mar-2011 19:19:32] PHP Notice: Undefined variable: wpdb in /homepages/staging/wp-content/plugins/buddypress/bp-core/bp-core-wpabstraction.php on line 13
