I've been watching a 'debugging proposal' thread and eventually
started playing with the idea of porting debug.rb to use a new event
tracing API.

You can find the first version of ruby-debug extension here:
http://datanoise.com/articles/2006/07/07/faster-debugger-for-ruby

Beware though that I didn't have much time to test it.
Comments are welcome.

-- 
Kent
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http://www.datanoise.com