There is a new filesystem on FreeBSD 8-CURRENT, which was imported from NetBSD. It's name is tmpfs.
It is marked as highly unstable (and the module will tell you that, every time you mount a tmpfs), but i'm successfully using it for over two weeks without any problems. To mount that file system, you need to load the tmpfs module or you'll have to compile it into your kernel.
You can then mount the filesystem via
mount -t tmpfs tmpfs /mount_point
or you can place the line
tmpfs /mount_point tmpfs rw 0 0
into your /etc/fstab. Have fun