Well, I know exactly where you are coming from.
Let me make a suggestion ... based on my experiences.
I WILL NOT shoot a wedding with a single card camera because I refuse to carry around two cameras at once for back-up. Being able to capture to two 16 gig CFs at once is cheap insurance against card malfunction.
As a wedding professional, assuming you are the first line shooter, perhaps think of the A900 as a suppliment. Shoot the "must haves" with the dual card camera, and portraits/candids/isolating guest from the background shots with the Sony.
I sometimes take one with me along with the dual card camera (previously a Canon 1DsMKIII, now a Nikon D3X or D3 both of which are FF and shoot to two cards). When I do that, I only take the Sony A900 with just two lenses ... the 85/1.4 and 135/1.8. All the zooms like a 14-24 and 24-70 are Nikon ...plus one fast Nikon lens just in case (like a 50/1.4 ... if it were the Canon I'd take the 35/1.4L)
Since you have a Canon 1DMKIII you have low light work covered ... the Sony isn't that great in low light when shooting on the fly like at a wedding ... and doesn't focus as fast as your Canon in low light (with the exception of when using the 85/1.2MKII-L). But for the most part the RAW files come out of the A900 camera pretty much there, and can be processed to jpgs in the PS Image Processor in just minutes. Same for Lightroom.
If you want to use flash with the A900, you can buy a converter adapter to fit Sony's propritary flash mount and just use one of your Canon flashes set on A ... works fine without buying yet another flash.
Just a thought ... you get to have your cake and eat it too