I've started a small repository for AMD64 builds of Debian and Ubuntu packages. I found that there are a quite of lot of debs out there from various sources which have been built for i386 architecture, but not for AMD64. It is possible the i386 debs will work on an AMD64 Ubuntu I think, but you'd have to 'force' the install, and apt-get won't find any i386 debs anyway, so it's a much better idea to rebuild the software.
As I find software I need but which doesn't have an AMD64 build, I'll rebuild it from deb sources and upload it. So far I've only done kio-apt and monotone. I'll put a proper page up listing what I've got soon.
For now, you can get the packages using apt-get and adding these lines to your sources.list:
deb http://lukeplant.me.uk/debian ./
deb-src http://lukeplant.me.uk/debian ./
I've only tested them with Ubuntu, which probably corresponds to Debian sid (unstable). I may also accept requests for building AMD64 packages for those who don't know how (or can't be bothered!).
By the way, the instructions for building monotone warn that you'll struggle to build it with 128Mb of RAM, and they weren't wrong! I found the g++ compiler using up to 170Mb to compile that thing!
I've now started playing with it for the rewrite of cciw.co.uk which really needs to be done. However, I don't think it will really be a distributed development project (which is what monotone is for), unless there are some more CCIW people who know PHP and are interested in hacking on this. Anyone?