Since there is a lot of buzz about Visual Studio 2008 release, I thought I would stand and say about ReSharper in relation to this fact.
ReSharper 3 can be installed with Visual Studio 2008 and works fine, unless you are using C# 3.0 new features, like lambdas, LINQ, extension methods and such. This constructs are not parsed by ReSharper 3, which was developed to support C# 2.0 only. It is not only highlighting which doesn't support C# 3.0, it is all the core and code intelligence. For example, rename refactoring will not find usages of extension methods and will not update them.
There also could be some glitches, even if you don't use new C# 3.0 constructs. This is due to the fact that C# 3.0 compiler is always used in Visual Studio 2008 C# projects, regardless of target framework. It it is not widely discussed, but changes in language are wider than just several new features. There are differences in type inference in generics and candidates lookup for binding, to name few. If you happen to hit this special cases, ReSharper could behave incorrectly.
If you are really-really going to immediately jump C# 3.0 wagon, you can disable highlighting (Options / Code Inspection / Settings) and switch to Visual Studio native intellisense (Options / Environment / IntelliSense / General). This will help a bit, but still you cannot trust find usages results, refactorings and many other features.
There was also reported weird problem with Visual Studio 2008 and ReSharper installed on x64 computers - opening Visual Studio's Find dialog crashes. We are currently trying to reproduce this problem. If you experience this problem, please tell us!
Some people tend to ask for quick-and-dirty hack for ReSharper 3 so that it just parses the code and don't do anything intelligent with C# 3.0 code. It is not possible. Details are not important here and are pure technical.
ReSharper 4 is in very active development. Its main purpose is to support C# 3.0 in all of its beauty. This means not only parsing and code intelligence, but also new analysis, refactorings, context actions and quick fixes. We are concentrated on making your development experience with C# 3.0 as smooth and pleasant as possible.
Currently, we have support for implicity typed variables and arrays, extension methods, object and collection initializers and automatic properties. As soon as we complete support for lambdas, queries and anonymous types, we will open Early Access Program. We plan to achieve this goal in January, 2008.
From this point you will be able to download EAP or even nightly builds and try full power of ReSharper 4 with your new C# 3.0 code.
In regards of upgrade policy, we are currently in the process of deciding upgrade cost, who qualifies for free upgrade, or if we want to do something special about this release. I will post about it as soon as I can.
As for release plans, we are aiming at 2008'Q1, hopefully sometime soon after Visual Studio 2008 launch event in February. ReSharper 4 will be available to general public via Early Access Program at least for 2 months before release.
If you have any questions, do not hesitate to ask in comments.