So I was having huge problems this morning trying to install Silverstripe on to a shared host server where it was complaining that it did not have permission to write to the temporary folder "/tmp/silverstripe-cache" as part of the "Is the temporary directory writeable?" test.

It took a bit of time to dig around and find the answer, but this old post had the solution:
http://silverstripe.org/archive/show/727

Basically, for a shared-host situation, simply create a folder called 'silverstripe-cache' in your www or root directory and the installer will use that instead.