In wpmu RC1.5 when install, in phpmyadmin 2.9.2 in tables: wp_1_comments :
Indexes: Documentation Keyname Type Cardinality Action Field
PRIMARY PRIMARY 0 Edit Drop comment_ID
comment_approved INDEX None Edit Drop comment_approved
comment_post_ID INDEX None Edit Drop comment_post_ID
comment_approved_date_gmt INDEX None Edit Drop comment_approved
comment_date_gmt
comment_date_gmt INDEX None Edit Drop comment_date_gmt
! More than one INDEX key was created for column comment_approved <------ Warning
click in edit comment_aproved and: ("PRIMARY" must be the name of and only of a primary key!) <----- Warning