Upload & Sell: Off
Hi Rich, first off sorry to hear you're feeling rough. Not fun.
I experienced something very similar at the start of the season. Was drastically unwell the week before a wedding and was sure I wasn't going to be able to do it. I dragged myself to their rehearsal, sweated my way through the whole thing, nearly vomited twice and then explained face to face at the end of it that I may not be able to shoot tomorrow. They were understandably concerned but I also explained that I had lined up a replacement.
In the end I woke up the following day feeling much more stable and I did the job myself. It was hell, but I made it.
Here's what to do. Get on twitter, facebook and speak to any local photographers you rate who might be able to help. Put the call out now and see if you can line anyone up. Agree to pay that photographer your fees (or whatever it states in your contract). Then, once you've lined that up, speak to your clients and explain the position. Reassure them that their wedding WILL get shot, and shot properly. Also reassure them that if there is anyway you can make it through the day you will, but it's important to you that they have a proper photographer and if you can't do your job properly it'd be better for them that replacement X stands in. Explain that they are ready and waiting and that they have nothing to worry about. Other than a different face behind the camera, from their POV things will change as little as necessary. Then see how you feel on the day and be ready to go if you're feeling passable.
If you can't line up someone else, then in a imperfect situation stick to your contract and be ready to return the fees.
Finally when and where is the wedding?