Here is a NAnt task to use the Microsoft Research ILMerge software in your build scripts :
http://www.evain.net/public/ilmergetask.zip
You can take a look to the source here :
http://www.evain.net/public/ilmergenant.cs
To build it yourself, you just have to configure the path to ilmerge.exe in the .build, and run nant.
ILMergeNAnt is builded againt ILMerge 1.0.1661.24486.
Due to license restriction, I can not include ILMerge here, so if you don't already have downloaded it, you can get it
here.
If you still wonder about what ILMerge is, let's go
here.
Update
Thanks to Dominik Guder, here is a new version of the ILMerge NAnt Task :
http://www.evain.net/public/ilmergetask.zip