Effective May 11, 2009, the price of mailing a standard 1oz. envelope via First-Class mail will increase from 42 cents to 44 cents. In addition, the price of mailing a 2oz. envelope will increase from 59 cents to 61 cents. The 2oz. mailing category is what most wedding invitations fall into. To accommodate the heavier weight of an invitation, the USPS is issuing a 61 cent 3-tier wedding cake stamp, which can be seen here.
With that being said, never try to "guess" your postage amount for your wedding invitation. Always take a completely packaged invitation to the post office to be weighed for an accurate postage amount.
One final note, please pay close attention to this increase if your RSVP date falls around May 11th. When attaching postage to your RSVP envelopes, do not take any chances - place a "Forever Stamp" on each envelope to cover yourself. This way, you will be certain to receive all of your RSVP's.