@asnelt: I believe /tmp is on a tmpfs. I think /var is not. Realisticly, parts of var could be in RAM (and might be for the next release) but there are also parts that see significant amounts of large files, such as /var/cache/apt/archives.

@qole: normally, desktop environments don't come with init scripts. Until the next release, you might be stuck modifying the init script that launches hildon to launch xfce.