Getting Code Ready for Ruby 1.9
In Getting Code Ready for Ruby 1.9, James Edward Gray II is attempting to put together a comprehensive blog post on the various things you need to do to migrate your Ruby 1.8 code over to working on Ruby 1.9. So far, James only looks at a handful of potentially sticky areas, but commenters have already started chipping in with suggestions, including how to rewrite the BlankSlate class for Ruby 1.9.
Further afield, Sam Ruby has written Porting REXML to Ruby 1.9 where he rattles off a laundry-list of things he's had to do to get REXML ported to Ruby 1.9. This will also be a useful read for those migrating code to 1.9.