Find out the user's IP address given their UID?
If you have statistics module enabled, the "accesslog" table contains the IPs for all your visitors.

SELECT hostname FROM accesslog WHERE uid = %UID_FOR_YOUR_USER% LIMIT 1


If you do not have statistics enabled, I advise you to leave it off, even for this result, because enabling it, causes a severe performance-penalty.

In that case, you can use sessions table:

SELECT hostname FROM sessions WHERE uid = %UID_FOR_YOUR_USER% LIMIT 1


The benefit of the first over the latter, is that the first will always show you the latest hostname, whereas the latter might not always be up to date.