We lost an engineer recently because of a desire to change careers. I still see him once a week - as he still works one full day a week for us as a contractor. It has worked out very well in that the knowledge has not been lost and provided continuity in that area. I wish I could work him more time - but he likes things they way they are.

This is probably a fringe case - and I do not recommend you pursue it unless you are in excellent standing with your manager and are not currently overloaded with responsibility. This type of transition doesn't work if many people are solely dependent on you - which may give you something to work towards - others cross trained directly in your task - so you can have time away without impact to the company.