Z:gnu-www-ja-gpl-faq--c592e3-Version 3 of the GPL allows th/en

Version 3 of the GPL allows this; see option 6(b) for the full details. Under version 2, you're certainly free to offer source via FTP, and most users will get it from there. However, if any of them would rather get the source on physical media by mail, you are required to provide that.