If we change ActionView::Base.erb_variable, cache_fragment doesn't work and some javascript functions also, because they are checking for the existence of '_erbout' instead of the variable set in erb_variable.
It's just a small patch, that fix javascript functions and send erb_variable to Erb.new().
I also added the line below in fragment cache tests setup to assure that they are working with any variable set:
ActionView::Base.erb_variable = '_buf'
I am curious to know why you need to change the value of ActionView::Base.erb_variable ? I'm more inclined towards removing that configuration option altogether.
I'm using it with Erubis and it's very handy!
If the point is accept many template handlers, that flexibility is needed. Otherwise I would need to hack any new template installed.
Fair enough.
Cannot apply the patch as I'm gettin about 25 actionpack test failures/errors 
This is strange, I wasn't probably running all tests. I will take a deeper look into it!
Oh crap! =P I forget to set a teardown in caching_test.rb, at the end of the file.
def teardown ActionView::Base.erb_variable = '_erbout' end
I've attached another file in Lighthouse. =)