Why?
If your a developer you will have the project or the dll on a folder or location that may not be the same as the location where it will be on the client machine, even if you use 3rd party assemblies you will see that the assemblies usually get on 2 places, 1 on the directory where you installed the 3rd party application and sometimes in the GAC, In VS.NET 2003 and 2005 also the list that appears when you do a add reference isn't a list of assemblies in the GAC instead is all the .net framework assemblies + whatever is defined on the registry. So if you want your Assembly to appear on that list you will need to add the following:
Open Regedit
In VS.NET 2003 go to HK_LocalMachine\Software\Microsoft\.NETFramework\AssemblyFolders
under this key just create your own with whatever name u like i.e. MySharedAssembly, now modify (Default) and enter the path to the directory where your assembly is located, this will make that allassemblies on that path get's displayed on the add refrence screen.
Now for VS.NET 2005 registry path is HK_LocalMachine\Software\Microsoft\.NETFramework\v2.0.50727\AssemblyFolders

Hope helps you a litle more.

>I was just finding that out... That doesn't seem very straightforward to me.