Instead of looking at going part-time, have you considered taking on more responsibilities? I'm just thinking, if management is told that you are wanting to go half-time and can continue to get your workload done, maybe they have other things that they need help on and didn't know they could come to you and they could get upset that instead of looking at helping out you're talking of leaving (even if only half-time).
If there isn't room to expand and grow your role, then I'd take it to them as a money saver idea. Like Bethalize said, tell them you can work part-time, saving them money, and are willing to work hard enough that they wouldn't notice a difference in the office (except for your smiling face not being there half a day

).
I'm sure there are other thoughts/ideas out there, so hopefully someone can offer other advice.
CountriGal
Deskdemon Editorial Board Member