I was able to correct this behaviour. I know it's a weird method, and it's not very clean...
In my case, there's only one appointment by room, so I wanted to use all free space.
To do so, in the AppointmentCreated event, I added (C#) :
WebControl) e.Container.Parent.Parent.Parent.Parent).Width = new Unit(100, UnitType.Percentage);
It works on IE7 and Firefox 2. Well, I've seen that Q2 is released tomorrow, you may not need that, but still.