If you preview messages in the Reading Pane or if you have the Reading Pane turned off, but you open the meeting notification (even if you haven't accepted or declined yet), then the meeting will appear on your calendar as Tentative.
In Outlook 2003, you can also choose to auto-accept meetings. To do this, select Tools | Options | Preferences. Click on the 'Calendar Options' button. Click on the Resource scheduling button. Check the box for 'Automatically accept meetings and process cancellations.' Meetings will only be auto-accepted when Outlook is running (this means they will not ibe auto-accepted f you are accessing your calendar via Outlook Web Access).