Mine

msf > db_connect REDACT:REDACT@localhost/msf3
[-] Error while running command db_connect: Failed to connect to the database: uninitialized constant MysqlCompat::MysqlRes

Call stack:
/Users/savant/pentest/msf3/lib/msf/ui/console/command_dispatcher/db.rb:1768:in `db_connect_mysql'
/Users/savant/pentest/msf3/lib/msf/ui/console/command_dispatcher/db.rb:1614:in `send'
/Users/savant/pentest/msf3/lib/msf/ui/console/command_dispatcher/db.rb:1614:in `cmd_db_connect'
/Users/savant/pentest/msf3/lib/rex/ui/text/dispatcher_shell.rb:331:in `send'
/Users/savant/pentest/msf3/lib/rex/ui/text/dispatcher_shell.rb:331:in `run_command'
/Users/savant/pentest/msf3/lib/rex/ui/text/dispatcher_shell.rb:293:in `run_single'
/Users/savant/pentest/msf3/lib/rex/ui/text/dispatcher_shell.rb:287:in `each'
/Users/savant/pentest/msf3/lib/rex/ui/text/dispatcher_shell.rb:287:in `run_single'
/Users/savant/pentest/msf3/lib/rex/ui/text/shell.rb:143:in `run'
./pentest/msf3/msfconsole:130
msf > ruby -v  
[*] exec: ruby -v

ruby 1.8.7 (2009-06-12 patchlevel 174) [universal-darwin10.0]
msf > gem -v
[*] exec: gem -v

1.7.2
msf > gem list -d mysql
[*] exec: gem list -d mysql

mysql (2.8.1, 2.7)
    Author: TOMITA Masahiro
    Rubyforge: http://rubyforge.org/projects/mysql-win
    Homepage: http://mysql-win.rubyforge.org
    Installed at (2.8.1): /Library/Ruby/Gems/1.8
                 (2.7): /Library/Ruby/Gems/1.8

    This is the MySQL API module for Ruby
msf >