After installing ISA server on an internal network, I noticed that file upload via FTP no longer worked.  It took way too much Googling to find this link, which explains the simple solution:

• Right-click the 'SBS FTP Outbound Access Rule' (in a default installation)
• Choose 'Configure FTP' from the bottom of the context menu. 
• Deselect the 'Read Only' box on the dialog
• Restart your FTP client application, and voila!