Microsoft ISA Server blog has posted an article on this topic that goes into details on how to deal with this issue of not being able to check out a document in MOSS 2007. Here’s an excerpt:<\/p>\n
“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<\/em>. Although this can be a good argument it doesn\u2019t prove that the issue is on ISA Sever. The reason why it doesn\u2019t prove is because most of the time while publishing MOSS 2007 through ISA Server 2006 the Alternate Access Mappings is controlled by MOSS. This is a key element in this type of publishing scenario, so before we move further on this issue I strong recommend you to read the following article: Plan alternate access mappings (Office SharePoint Server)<\/a>. This article has all the concepts that you need to plan your AAM without hurting your publishing rule through ISA Server.”<\/p>\n