I realise this kernel isn't final, but since this was mentioned in the change log (http://www.virtualbox.org/wiki/Changelog), I thought I'd bring it up.
When I attempt to compile the kernel module with 2.6.27-rc3, I get the following
# make
...
/usr/share/virtualbox/src/SUPDRV.h:104:30: error: asm/semaphore.h: No such file or directory
/usr/share/virtualbox/src/linux/SUPDrv-linux.c: In function 'supdrvOSGipResume':
/usr/share/virtualbox/src/linux/SUPDrv-linux.c:1331: error: too many arguments to function 'smp_call_function'
make[2]: *** [/usr/share/virtualbox/src/linux/SUPDrv-linux.o] Error 1
make[1]: *** [_module_/usr/share/virtualbox/src] Error 2
make[1]: Leaving directory `/usr/src/linux-2.6.27-rc3'
make: *** [vboxdrv] Error 2