I'd personally recommend against running multiple VM platforms just because it's more management overhead and more documentation overhead. No reason it's horrible to do it though.
I wouldn't use Virtualbox for a server to be virtualized just because in my experience Vbox is more of a workstation-oriented platform, while VMWare and Xen and Hyper-V are more of a headless-multinode VM platform. But if it works for you, more power to you.
In all it's relative to what you're most comfortable with. If you're going to be comfortable with the overhead of managing multiple platforms and well as getting backup solutions in place you should be all set. Randolph Potter mentioned the same hard disk format for easier migration; he has a point. VM's make it more crucial to have backup plans in place since VM servers are a single point of failure.
You might be just as well off, though, if you have a backup solution in place that treats the virtual servers as real servers. Then you can do a "virtual bare-metal" recovery of your server in place; test it, though, as you don't know how drivers will react if you switch platforms for some reason.