I'll post a more thorough rundown on my site, but here's the basic idea:

Add a "lock" flag (column) to each table that can be edited by more than one user. Also add a "edit started" timestamp column.

When someone clicks on "edit", set that record's flag to "1" if and only if its current value is 0. Now let that person access the edit form.

If the flag is already set to 1, throw up a notice that someone else is editing that record.

When the 1st person submits the change, flip the flag back to 0.

Person 2 can now edit the record.

To address the "stepped away" issue, run a scheduled task every 30 minutes or so. Look for records with a flag = 1 and an edit start time > 30 minutes. Adjust the timing as you like.

I did this with an application for US Airways and it worked out very well for all involved.

I haven't tried this but I don't see why you couldn't use AJAX. On edit pages you could have an AJAX function touch base with the server every 5, 10, 30 seconds (what ever seems the right balance). If more than a few of these are missed consecutively, the flag it reset and the record released. The user would have to move off the edit pages to lose their lock. You would probably still want to run a clean up routine like above as a backup.

@Magnus, I'll have to take a look at that.

@Matt, Do you know of any examples?

@Chris: checking usernames would work just fine. Also, you would want to have an Admin user that could go in an unlock individual records or run the bulk unlock whenever they choose.