"Troubleshooting Sharepoint/MOSS 2007 publishing through ISA Server can be really challenging, mainly because most of the times the argument is: but it works just fine internally. Although this can be a good argument it doesn’t prove that the issue is on ISA Sever. .."

Check it out the complete post that born from a nice collaboration between ISA and Sharepoint Team:

http://blogs.technet.com/isablog/archive/2008/10/02/unable-to-check-out-a-document-in-moss-2007-published-through-isa-server-2006.aspx