SQL Query Question concerning math between rows

[jon silver]

I have a table with 3 columns: uniqueindex, userid and timestamp
There is an entry for each time a user submits a post.

I wanted to find
1. The average time between posts
2. users with posts within 10 minutes

is there any super type of query ? or do I need to write a function that cursors through each row by userid?

thanks,
jon