Okay, so first of all I didn't realise that /tmp is limited to only 128mb, regardless of RAM size for a VPS, which is a bit annoying. But I recently had dhapache fill my /tmp directory with seemingly cache related files, but filling it to such an extent that PHP was failing to handle uploaded files because there was no room for them.
Now, I've created a new location for PHP uploads under my home folder which has solved the problem, but is it normal for /tmp to fill up so much? At this precise moment it's not as bad as it was, but I still have nearly 100mb of stuff in there at all times, is this normal for such a limited /tmp allocation?
I think Dreamhost really ought to provide a second temporary folder under ~/tmp or something, with a default cron command to keep it tidy, with PHP and other services configured to use that location instead. Currently PHP uses /tmp by default for file uploads and also for saving sessions, which of course is no good as it means if /tmp is filling up thanks to all the dhache cruft inside it then sessions will only last a matter of minutes and uploads will fail.