I don't think there's a way of doing this at the moment, but it would be really handy for me.
I'd like the ability to use 'virtual' monitors. That is, to run in windowed mode but have several windows open per vm, simulating several monitors per machine. I'm aware of Fusion's support for true multiple monitor setups and I use it on a dual monitor setup, however a lot of the time I'd like to run three or even four 'virtual' monitors - each monitor represented by a new window in Fusion. Is handy for the software development and testing I do. I also don't always want to devote an entire monitor setup to a guest OS, but would still like the guest OS to believe it has several screens available.
As I say, I'm pretty sure this isn't possible at the moment. If there's some other way of achieving it then please let me know, but virtual monitor support would be a decent addition to Fusion I think.
Cheers,
Ian