From what I see Nothingham is exactly right.
You see 90% of your applications your going to deal with are autotooled and will compile across many different platforms. Most of it is very automated..
this is the main reason that Debian still uses XFree86 and hasn't moved to X.org. It's because XFree86 is a monolythic thing.. no real automated way to install and test it. Many applications are dependant on it's libraries and it is non-modular. It makes it a pain in the @ss to upgrade it between versions. (luckly with X.org they are working towards a autotooled and modular free X server, which will make upgrading between versions much easier, and will benifit not just Debian but all distros)
And like it was stated before once you get a application that is written well enough to compile cleanly accross x86, x86-64, PowerPC, and Itanium it's pretty much garrenteed that it will work just fine on something like Mips or Arm.
(and if you think that stuff like ARM is very obscure, realise that XScale which is used in the majority of powerfull handheld devices is Arm archatecture)
One of the major benifits of Linux/Unix over something like Windows or VMS is that it's cross platform. When Windows has to move over to x86-64 it's a pain in the rear, but all this supporting dozens of different platforms is what enabled Linux to have a relatively stable AMD64 Suse distro the day that AMD started selling it's next gen platform to the public. Windows still hasn't got a non-beta AMD64 version yet, which was originally scedualed to be released 2 years ago.
What screws over Debian's release scedual is feature creep.
Just one more upgrade. Can we get KDE 3.4 included into testing, that would be realy cool!
That sort of thing.
If Debian would stick to it's guns about what is and what is not going to be included in the next stable release we wouldn't be discussing this issue.
The cross-platform thing is a red herring. It causes issues with the Debian Installer and such, but that is about it.