I ran into this a while ago so I thought I would post it here too.
Add a new string value named "Guides" to [HKEY_CURRENT_USER]\Software\Microsoft\VisualStudio\9.0\Text Editor. Set the value to "RGB(128,0,0) 100" and you get a nice red vertical line after column 100.
Update: THis functionality has been removed from VS2010 but you can use these plugins: http://visualstudiogallery.msdn.microsoft.com/en-us/0fbf2878-e678-4577-9fdb-9030389b338c and http://visualstudiogallery.msdn.microsoft.com/en-us/7f2a6727-2993-4c1d-8f58-ae24df14ea91
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