For the last 6 years of so, Microsoft have launched 3 different framework versions (1) .NET Framework 1 (2) .NET Framework 2 and (3) .NET Framework 3 (to be released soon) . http://en.wikipedia.org/wiki/.NET_Framework
The advice would be to get certified on all, as they're released, learn about them and get certified as early as possible. Makes sense right. That's if you were around when it just came out, but what if you've fallen behind (i.e. still coding in version 1.1 and haven't started on 2.0 yet) can that approach really work?
There's an issue circulating among my peers and that is, going forward, which frameworks and their supported technologies do I get certified on to be a marketable developer. In my opinion, get certified on all of them!
The logic for backward certification
Currently our company is pushing to do some new developments and this will be done on .NET Framework 3.0 (WCF) and some done in .NET Framework 2.0 whilst still maintaining tonnes of code we have currently in .NET Framework 1.1.
Companies may not have the funding to upgrade their stable applications to target different frameworks, so v1.1 might be around for a very long time and someone has to maintain it.
Microsoft has made the certification track clear for everyone. After completing the 3 exams required to become a Microsoft Certified Application Developer (MCAD) a developer can easily upgrade to the Microsft Certified Professional Developer (MCPD) by doing two additional exams.
In my opinion it's a quick path to the new versions whilst being very much in touch with the previous version. The key is to complete these ASAP and then be ready for the release of the new Microsoft exams on the newest framework.