I run a highly modified cart and have both shipping and download products. I am not using attributes or the download mod (products require registration, no need to protect the download files).
Anyway, after searching the boards here without finding an answer and then a bit of head scratching I realized a simple mod could make orders that have a zero weight (ie, all products were download only) skip the shipping page. Since I had not found this answer in my search I thought I'd post my solution for others.
NOTE: this contribution will cause orders in which all products have no weight to SKIP THE SHIPPING PAGE. This is intentional; it's the goal and only purpose of this contribution :)
Possible uses for this contribution are for download product, a donation website, or any other sales where there is nothing to ship and so no shipping information is required. This removes a step in the checkout process for these customers, which is always a good thing.
Top 4 Download periodically updates information of Zero Weight Skips Shipping script from the developer, but some information may be slightly out-of-date.
Our script download links are directly from our mirrors or publisher's website. Zero Weight Skips Shipping torrent files or shared files from free file sharing and free upload services, including Rapidshare, MegaUpload, YouSendIt, MailBigFile, DropSend, HellShare, HotFile, FileServe, MediaMax, zUpload, MyOtherDrive, SendSpace, DepositFiles, Letitbit, LeapFile, DivShare or MediaFire, are not allowed!