I upgraded to 1.5.1 yesterday (5 minutes!) and today started updating and activating plugins. I re-installed the Geo plugin and, on activation, I get errors on the admin pages...
Warning: Cannot modify header information - headers already sent by (output started at /.../wp-content/plugins/geo.php:751) in /.../wp-admin/admin.php on line 6 (also get it for line 7, 8, and 9)
De-activating the plugin makes all well again.
I admit openly to not being a PHP whiz (perl is my primary tongue) so I was curious to see if anyone else was having the problem or had an idea where to start looking.