I used to work with eclipse for nearly all the languages I need. I'm asked to work on a tool developed in C# and so, I would like to stay in the same familiar environment.
I've found the improve's plugin but its last release is from 2004 and .NET 1.1 which is quite old. Is there a newer plugin to program in C# within eclipse or am I forced to take a look at VS?
I fear, that there is no good eclipse plug in. Try http://www.monodevelop.com/Main_Page or http://www.icsharpcode.net/OpenSource/SD/. And the free visual 2008 express editions are worth a look.
Emonic is worth a look as Jasper suggested. I've installed it in the past myself, but over a year ago. Checking the change logs on the site, it does not appear that they have had any new releases since then. The worst thing about it is that it does not supply a debugger or any refactoring tools. I've found that if you're going to work with Microsoft products it's best to eat the whole hog.
You will have a learning curve getting into visual studion from eclipse, but it will probably save you some time working out the nuiances with a product trying to build .NET code.
Visual Studio is a very nice environment to work in, the express editions are free so my suggestion would be to take the oppertunity and have a look at the VS dev environment.
MonoDevelop just released a Windows Beta, and it's looking very good. It's a cross platform C# IDE. It may be of use.