Your system is 32 bit (i386 arch) because you installed an amd64 kernel but still have the 32 bit userland. If you want a true 64bit system, back up your files and reinstall.

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The fact that the processor has 64 bit capabilities, doesn't mean that the installed system is 64 bit. And using a 64 bit kernel doesn't make a 64 bit system either, since the rest of the programs are compiled for i386 (32 bit). ia32-libs is not in the repository probably because it is intended to run 32 bit executables on truly 64 bit systems (and you're still using a 32 bit system), while amd64-libs is the opposite (it allows to run 64 bit executables on 32 bit systems).
Anyway, if you want a complete 64 bit system, you must reinstall, as Caravel said.

BTW, I tried installing a 64 bit kernel on my 32 bit Debian install, attempted to install the 64 bit nvidia driver and had the same error you had, then I installed amd64-libs and the installation program ran fine this time.

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I had the same problem and that helped me too.
The Debian Squeeze installer automatically installs amd64 kernel and 32 bit apps which is a bit scary.

Can I expect further difficulties when I keep the 64 bit kernel and run with amd64-libs ?

I'm not sure if it will cause problems, because I only tried it for a short time during the past days, but I prefer either a complete 32 bit system, or a complete 64 bit system (with mixed libs for compatibility reasons). If you're sure that only your kernel is 64 bit, you can always install the i686 one and uninstall your current kernel (after you boot the i686 kernel, of course). Check the repos for linux-image-2.6-686, that's the one you need.