That's how Skype's option plan is described in this piece by Felix Salmon. I've seen option plans that have repurchase rights in them. They used to be more common twenty five years ago when I entered the venture capital business. The theory was that employees would have to stay until the exit if they wanted to keep their equity (be in it to win it). But in practice, once employees realized that was the deal, they were actually incented to leave because they didn't trust that the equity they were vesting would ever produce a payday for them. So they went elsewhere and created value for an employer with a better deal.
Today, the "market" deal in employee option plans is that employees have to exercise their options within some period after leaving (typically 90 days). This is a better deal for the employee than a repurchase right but can still create hardship for employees as it may cost real money to exercise and there are often tax issues with exercising options. This requirement to exercise upon departure is a big reason why the secondary market in employee common stock has taken off. Employees who leave companies need to sell some of their vested stock to come up with the cash to exercise and pay taxes associated with exercise.
This is a tricky area. Companies feel that employees who stay and work to create ongoing value should have a better deal on their vested options than employees who leave and go to work elsewhere. I understand that point of view. But it is also true that your employees need to feel that the options they are vesting are going to be worth something and that they will be able to keep them when they leave.
Companies also want to control their stock and keep it off of secondary markets where they can end up wtih shareholders who they don't know. The requirement to exercise within a short period of departure is in conflict with the desire to control the cap table.
I believe this whole area of "what happens with the options when the employee leaves" needs to be rethought in light of where we find ourselves right now in startupland. I'm not sure I have any particularly good ideas but I know that the way we do it right now is problematic for everyone.