Works well here - Kubuntu 10.10 64-bit Laz. 0.9.31 (29271) + fpc 2.5.1
I add Dialogs to my "uses" section and f.i. this: MessageDlg('!fgfg!','Str',mtError, [mbOk, mbCancel],0); works well.
Note: I have no messages.inc, I have only .../lazarus/lcl/include/messagedialogs.inc