I might have blogged about this before, but I could not find my own blog post, so why not blog it again.
Since the recent CodeRush 10 beta, which works in VS2010 RC finally, there is no more Right Margin line that can be enabled under Options->Editors->Painting. I also tried to enable some modules, but I cannot see the margin line anymore (works fine with CodeRush 9).
Well, in Visual Studio you can also enable a margin line with a registry entry at:
[HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\9.0\Text Editor]
"Guides"="RGB(64,64,192) 79"
This will produce a blue margin line right before the 80th column. It looks good on white and black backgrounds, but you can of course change it easily to any color and position. If you don't want to go into regedit, just execute this handy RightMarginLineVS2008.reg (280 bytes) file.