There have been lots of articles about the Zynga "taking away employee's options" brouhaha (another one today in the NYTimes). One thing that bugs me is that many of these articles don't seem to distinguish between vested and unvested options. The difference is VERY important.
When you work at a startup, think of it as you get paid bi-weekly or monthly in two ways: cash (your salary) and options (the options in your option plan that vest that month). In theory you could actually get option certificates issued to you bi-weekly or monthly the way you get cash sent to you for your salary, but that would create a lot of extra paperwork. So instead someone invented the idea that the employers tells the employee "you get N options over 4 years and they vest every month" or something like that. Maybe to make things clearer they should have said "you get M options every month you work here" (where M would presumably be 1/48th of N). This, however, would create a planning complications for the CEO, who needs to carve out options from the "option pool" which usually involves lots of negotiations with the VCs.
So if an employer comes to you and says they want to take away *vested* options, that's like saying they want you to return salary already paid to you. I don't think they could do it even if they wanted to and if they wanted to I think we'd all agree that was highly sketchy behavior.
If, however, an employer comes to you and says they want to take away *unvested* options that's like them saying they want to reduce your salary going forward. That might be lame and unfair, and if anyone did it to me I'd probably quit, but it's very different than if someone tried to take away vested options.