Rick Brewster has put up a post on his blog about Paint .NET v3 which will now support MDI - my one and only gripe with the current version of this wonderful little tool.
I love this tool it is great to see what can be achived with Windows Forms and .NET in what is normally an application domain with a large memory footprint and slow response times however Paint .NET bucks this trend is always seems highly responsive.
If you havn't yet tried out Paint .NET give it a go. It is a tool I use 2-3 times a week when I just want to do a simple tweak to an image as it is quicker to load then Paint Shop Pro/Photoshop and easier to use but still has all the functionality you need..