Qemu + KVM is the future of open source virtualization

I haven't used KVM yet since my pony laptop doesn't have VT support, but I've used VirtualBox which is great. Its Seamless mode is amazing. After using it I've switched from VMWare Server to VirtualBox.

are there any serious (e.g. larger than I am) virtual infrastructure providers using KVM?

on the other hand, the linux guys /really/ don't like Xen; There are some open bugs in the debian paravirt xen kernel that have been open for quite some time. KVM unquestionably has a massive mindshare advantage when it comes to Linux developers.