According to reliable sources in Cupertino who our Macworld reporting team have been visiting now that the Expo is over, the Macbook Air was originally designed around Intel’s 45-nanometer mobile processors but was instead modified to accept slower-clocked “Santa Rosa/Merom” class Core 2 Duo CPUs at 1.6 and 1.8GHz.

It will take some time, but after the Macbook Pro is moved over to Penryn sometime in late Winter, the Air won’t be too terribly far behind. It will be a solid update, but we don’t necessarily think that readers should play the waiting game….

The first 45nm “Penryn” mobile Core 2 Duo processors, sometimes referred to as “Santa Rosa Refresh,” will be shipping at the end of the month but Apple won’t be satisfied with supply levels until somewhere in the late February/early March ballpark.

And, until the chips have been on the market for a time, their cost would have driven up Macbook Air’s price tag even further since the entry-level Penryn 2.1GHz chip costs considerably more than the 1.6GHz Merom/Santa Rosa now used in the entry-level Air.

We expect that except for the new Centrino chipset that the “SRR” is based around, and quite possibly second (or even third, if Apple decides to add an HD-DVD version — though we consider this less than likely) versions of the “USB Macbook Air Superdrive” featuring Blu-Ray or HD-DVD disc support…..very little else will change in the first revamp of the Air.

Read on for more specs, pricing, and details of the next generation Macbook Air….