I was having the following problem repetitively when using rake to run the tests on my ubuntu Edgy Eft machine:
rake aborted!
Lost connection to MySQL server during query
/config/../vendor/rails/activerecord/lib/active_record/vendor/mysql.rb:1127:in `write'

As I am using Radrails since the beginning of my ruby career , I have never found out before!!! Indeed, Radrails must use another driver than my plain ruby to access the mysql database.

It turned out that installing the libmysql-ruby solved the problem:
sudo apt-get install libmysql-ruby