I don't expect to see Apple include a 3G slot anytime soon in MBA, because most users won't find a good use for it. (Maybe in option? Who knows...)
Would love to see it though. I use 3G quite a bit with my iPad (and due to bad coverage, it's usually Edge down here), and I can imagine it can be very handy when having a mobile laptop like MBA.
For people who use the MBA as a real portable laptop (for example sales reps), I can imagine that they look for a cleaner 3G solution than MiFi's etc.