I recently sent in a support request about this. The getimagesize definitely supports SWF however it requires a static zlib to link against, having the zlib extension doesn't seem to cut it (as Dreamhost's PHP 5.3 install has Zlib extension 1.1 by default I think), this is because the vast majority of SWF's are compressed.
If you enable all error reporting when this function is called (which phpBB uses heavily by the way), then you get the following error:
It's definitely a change from 5.2, however nothing default in 5.3 accounts for it, so it appears to be something screwy with Dreamhost's 5.3 setup, as a default local install of 5.3 seems to be fine, even the zlib extension isn't enabled by default; this being because getimagesize doesn't require the zlib extension to actually be enabled.
I was hoping there'd be a way to work around it with PHP.ini directives, as I wanted to use PHP 5.3 due the much simpler phprc setup method for customising settings, but it's a non-starter until this is fixed