I think writing functional tests in a given-when-then style manner is a great idea. Tests tend to be clearer and more precise when written this way. There are quite a few tools too choose from. Cucumber by Aslak Hellesøy seems to have a lot of momentum these days and is an acceptance testing framework definitely worth considering.
However, the customer I work with has been using Fitnesse for a number of years now, which means people are familiar with this framework. So going for the new Fitnesse Slim framework seemed like a step in the right direction.
I believe that running fitnesse from inside of eclipse is a good idea. When Fitnesse is using my eclipse classpath I can modify my code without having to restart fitnesse. This gives me nice, short cycles and is major efficiency boost. A good colleague of mine, Morten Udnæs, had created a small start up class for Fitnesse in Eclipse. But when upgrading to the latest stable release of fitnesse (20090709) I found that I had to modify this class a bit – Uncle Bob has done quite a few refactorings over the last couple of releases.