I've done some research to see how the ZUG could host the ZSI without killing Michael's server/bandwidth.  Two options came up and they both involve 
ibiblio.  ibiblio is the current incarnation of the old 
sunsite at the university of North Carolina.  If you've ever downloaded a file from sourceforge.net you've been presented with mirror choices that include iBiblio.
Checking 
how-to-submit it looks like the threshold is low -- just make your submissions using the 
Linux Software Map description format and upload.  No muss, no fuss.  ibiblio already mirror numerous Linux 
distributions like Fedora, gentoo.org and others.  I think our feed will be accepted as well.  The ZUG could still host the ZSI itself but then delegate downloading to ibiblio which automagically 
mirrors the collection.  Sweet!
Another option, perhaps even more radical, would be to move the whole of the ZUG to ibiblio.  ibiblio is dedicated to making knowledge available for free.  Some sites already hosted there are 
groklaw.net, the 
Creative Commons project and 
others.  The 
collection policy implies the ZUG would be welcome since we compliment the alread existing collection policy for free and open source software.  See the 
FAQ for more information about what's available at ibiblio as far as hosting goes.
Now to be clear I'm not suggesting we consider moving as a reflection on the current ZUG or Michael's stewardship.  Both are FANTASTIC!!!  I'm just raising the possibility as a way to reduce costs and host the ZUG with other similar projects.  I'm also quite content to just host the files from the ZSI at ibiblio or not use it at all.  I'm presenting options and extending the dialogue.
--Aaron