In my experience, if you have a corporate card, you're expected to use it for everything that you possibly can. They issue the cards to be able to better track expenses and do the accounting/tax reporting more easily. However, when I've had corporate cards, the bill has always been paid by the company, not by me. I'd be pretty peeved if, like STBCypriot, I ended up floating my employer thousands each month without the benefit of getting miles for it. If that were the case I'd really make a stink, and probably just use my own cards as much as possible. At my current job I don't have a corporate card, and do float my company brief loans from time to time, but it's not egregious and I do get a boatload of miles out of the whole situation.

If it is a corporate Amex, you can just pay the $75/year to link the MR from the corporate card to your personal card.