I went to the regular London 2.0 meetup last night. Great people, interesting conversations and a little too much beer.
One of the Selenium developers, Jason Huggins demoed Selenium and Selenium-IDE on a Macbook notebook running the Parallels virtual machine software. He had several virtual machines running different platforms including Ubuntu Linux, Windows XP and Mac OS X with different browsers on each. Testing was triggered by checking in code to the SVN repository of Buildix virtual machine also running on the laptop.
The crux Jason's demo was that you could have a complete multiplatform web application test system running on a bottom level Macbook laptop (with extra RAM). The machine appeared to run very quickly despite the 5 or 6 VMs that were running. Jason mentioned that if you don't care about Mac browser testing (Safari) then you could set up a similar system using VMWare or Xen on Linux or Windows.
Selenium-IDE also looks seriously cool. It's a Firefox plugin that provides a complete environment for developing test cases. It can record your steps as you use a web interface and output to a variety of test script formats including a HTML stype sequential output, Python and Ruby. If using Python or Ruby scripts you can use all the features of the language such as conditions and loops. Selenium-IDE is going to make developing effective complete web tests so much easier (and fun!).