The rev B MBA is far superior to the original MBA. The original MBA was originally shipped late February or early March 2008. In October 2008, Apple introduced the rev B MBA which was totally redesigned INTERNALLY with all new components to eliminate all of the problems of the original MBA. The new revised MBA began shipping the first week of November 2008.
Using that model, BUT THAT IS ONLY ONE CYCLE, it was EIGHT months between revisions. So, the MBA is currently at 4.5 months or so. Apple could ignore the MBA for a long time as the current rev B MBA is absolutely AMAZING. It is a beautiful and extremely capable computer which is ultra portable.
However, the SL9600 Intel Core 2 Duo 17v 45 NM Penryn CPU is due out in April 2009. This is the "next" chip upgrade from the current SL9300 and SL9400 used in the rev B MBA 1.6 and 1.86 GHz models respectively. The SL9600 WAS projected to be a 2.13 GHz CPU last October... recently the Wikipedia site changed it to 2 GHz. Going from 1.86 GHz with a really nice 45 NM Penryn CPU with 6 MB L2 cache, that is a really REALLY REALLY minor update.
In addition, Apple has adopted some new technology in the unibody MB and MBPs that could show up in the next MBA. Apple has done away with the aluminum bezel and opted for glass display with black bezel. A lot of people think glass would add weight, and that could be true but doesn't mean the black bezel could not appear on the next MBA. Apple also has switched to glass trackpads that are buttonless in the MB and MBP.
Additionally, the new 17" MBP received a battery update which could be implemented in the next MBA to give it better battery life/performance. Quite a few MBA owners complain their MBAs only get 2.5 to 3 hours of battery life between charges. A new super density MBA battery could perform better, but would that happen if it adds weight to the MBA???
The RAM amounts Apple is using has changed, and many believe the next MBA could have 4 GB RAM. This is a completely plausible and most likely theory.
Finally, the MB and MBP now have a 256 GB SSD option, while the MBA still only has a 128 GB SSD option. The MBA uses a smaller 1.8" drive form though, and 256 GB SSD seems hard to come by in this size. Supposedly Apple had to wait for the 128 GB SSD to be "engineered" and made by Samsung before it could deliver the revised MBA. I wonder if this same issue will happen with the 256 GB or perhaps Apple already has a bigger one in order that could just change the BTO options for those who order a rev B MBA?
While Apple could give us a new MBA as early as April/May 2009, I think it is much more likely that it will be a longer wait. However, Apple is EXTREMELY UNPREDICTABLE... and the MBA is a high-end product that I think Apple is going to continually update like the MB and MBPs are updated twice annually, and have been for the last three years. So, one part of me says May 2009 is likely, another part of me confirms it could be eight months or the first week of July 2009 (this makes most sense to me as that is just perfect timing for SNOW LEOPARD), and finally the last part of me thinks that it will be more like September and the back to school season that the MBA will receive its update.
However, NONE of us truly knows the answer to this. You could buy a MBA this month, and next month the rev C MBA could be introduced. Also, you could hold off and it could be the end of the year before the revision C MBA is introduced.
I HOPE that it happens soon. I really want a silky smooth glass buttonless trackpad. I really want 4 GB RAM. I really NEED 256 GB SSD. At the same time, I don't know that I wouldn't just wait for Snow Leopard anyways if the MBA rev C was introduced today... the rev B is just so good... but I already own it, so I just don't know what to say to you.
Other than, life is too short. Buy the rev B MBA and be completely happy with it. Unless, you really feel you NEED a glass trackpad and 256 GB SSD options, as those are the two "guarantees" in the next revision C MBA in MY OPINION. But again with Apple, nothing is a guarantee.