If IS is important to you, then Pentax is a no-brainer. In-camera IS will save you a lot of money on lenses as you don't have to shell out a premium for the IS lenses like you do with Canon. A good example of this is the price difference between the Canon 17-55 IS and Pentax 16-50. Brand new, the 17-55 is $1030, while the 16-50 is $699.95. Another example is the 70-200/2.8--the Canon IS version runs $1699 while the Tamron and Sigma versions for Pentax run $699 and $799, respectively. You may not lose as much money on your Canon gear as you think you will.
If you make the switch, you might want to check around and get some other quotes on trading your Canon gear from places like keh.com, Adorama, and B&H before deciding what to do with it. You may also be able to get rid of it locally via Craigslist.
You might consider planning an outing with your friend who has the Pentax gear just so you can try his and he can explain how things work with the Pentax system. Hands-on experience will be valuable in your decision-making process.
If you haven't already done so, check out the Film SLR and SLR lens sections on the forum; you'll get a lot of valuable info on which film SLR bodies and lenses that people around here really like.
If you do decide you want to switch, I would wait until after the wedding in September before doing so, if I were in your shoes. You're already familiar with your current system, so assuming that this is a paying gig, why make it more difficult by having to learn a new system in a hurry?
HTH,
Heather