>> i can't expect whole IPv6 network will have MTU=1280.
>
>My outer IPv6 interface uses a MTU of 1280, too, and I can download files
>via FTP without problems.
>

In fact, the designers of IPv6 assert that you can make that 
assumption.  It's a requirement for IPv6 links.

		--Steve Bellovin, http://www.research.att.com/~smb