If I wanted one I'd get it now.
If a MkII comes out then it will be much more than you can get the MkI for so you have a different set of factors contributing to your purchasing decision.
Would you buy a MkII now for $2,200? Because even if they were available they wouldn't be selling for less than list, and wouldn't be for some time after release. Look after your MkI, keep the box and bag, receipt, warranty and paperwork and even if you want to upgrade in the future you won't lose that much.
Besides, lenses are for taking photos and imagine the images you will miss in the mean time.
If I wasn't in a rush I'd see what the rebates brought, but if you are not in a rush do you need it...............
Normally I'd agree but I wouldn't be shocked if the new 35mm 1.4 isn't sold at that big a premium due to the Sigma.