Where can I look up my update history?

You can read the history.log file in /var/log/apt.

Eg. less /var/log/apt/history.log.

In 10.10, Ubuntu Software Center has a list of all the updates you have downloaded in the past.

/var/log/apt contains a history of package installations. However, by default, it is managed by logrotate which compresses and ages out old entries.