I reckon the MBA upgrade will be a pretty quiet one, i5/i7 and maybe a thunderbolt on the 13", but I wouldn't bet on it. Maybe a bit more storage, but not a big increase if any. I'll be very surprised if there is anything bigger then the above when ever it does get upgraded... which hopefully will be pretty soon, as I'll have an order for a 13" maxed out Air put in straight away...
By quiet upgrade you mean not a big one or one that Apple won't hold a special event for? Because what you describe is far from being a minor upgrade. Going with a Sandy Bridge CPU would be already 3 CPU generations better than the current MBA.
The Thunderbolt port is a given on both models, since they already both have a mini DisplayPort output. It's the same connector, they don't even need to change the body, which is perfect the way it is anyway. Maybe we'll see 4GB RAM standard and a backlit keyboard, but I wouldn't hold my breath for bigger storage capacities.
Flash storage is the MBA's most expensive component, and Apple seems pretty serious about hitting mass market with the MBA. Notice how it's now used just as much as the MBP in recent promo material, if not more. I'm pretty sure they want to keep the price as low as possible, and make it the popular cheap model that the white MacBook used to be.
Plus, it's all about iCloud now. I have a feeling Apple won't increase its mobile devices' (iPhone/iPod/iPad/MBA) storage capacity as a way to kind of force you to store your music and pictures into the cloud, and keep hardware at a reasonable price since like I said Flash memory is pricy. I expect iCloud to continue growing in the upcoming years. Maybe it will hold pretty much all your data, including movies and applications. If that's the case then the current MBA's capacities will actually be overkill, since all you would need is the OS and maybe a little space for caching. Then all you would need is a cheap 8/16GB flash chip. The cloud is going to kill storage. They did it already with the Apple TV.
I know not everyone's a fan of depending on the internet to use their data. But that's what the future is shaping like (Chrome OS netbooks and such), and since we hear the next MBP will likely ressemble the Air (thinner and no optical drive), they have to keep the Air in a distinct class so maybe it will be the first to have integrated data connectivity and be the first Mac made for the cloud, until the MBP follows just like it will follow the current Air in the upcoming months.
Trust me, this iCloud thing is just a start. They're preparing you gradually for the next OS, which will likely be cross-platform across all Apple hardware and heavily dependent on iCloud. Lion is teasing it already with its iPad-like features. Maybe that "tiny" 64/128/256GB storage on the Air is the most it will ever have.