According to Wikipedia, the 64 bit version of Windows 8 will require 2 GB of RAM.
What should I make out of this number? Why is it so ridiculously high and what will the OS's memory footprint most likely be in practice?
It's 64 Bit – you only start seeing the advantage of it with more than 3.5 GB RAM anyway.. Windows 7 64 has the same requirements, and that's probably what it's based off.
The requirements also don't target the RAM the OS will be using all the time, it includes caching for performance, and quite a bit of headroom – you don't want to be swapping too much, after all.
To some extent, system specs are arbitrary – 32 Bit Windows exists primarily for legacy systems at this point, and most modern systems will have sufficient RAM (my 2007 era Core 2 desktops did!) – the 32 Bit version 'runs' on 128 MB, and it 'requires' 1 GB of RAM.
"Minimum requirements" means "This is the worst we tested it on, and really, we don't want to support people who want to run it on their toaster" and "recommended requirements" is the systems that the developers feel should run everything with maximum shininess. Other than enthuisasts, and people trying to win bets, they probably don't try the system on slower and slower systems 'to see if it will work'.
2 GB of RAM is probably required for all the advanced touch screen interfaces they are creating with Metro. 2 GB is actually not that bad as most netbooks these days even come out with 2-4 GB of RAM.
The average RAM is about 4GB on a decent machine with many new machines coming out with 6-8 GB as well. I just bought a new laptop, not a particularly expensive one, and it has 6 GB of ram.
Windows 7 also had similar requirements, so it's not really a big leap. This is Windows after all. :)
If you don't like huge footprints, may I suggest Linux? (Perhaps Ubuntu)
During the preview, running on a netbook -- so 32-bit -- the minimum footprint was 281 MB, compared to 404 MB for Windows 7. 64-bit would be more, but nowhere near double; either way, less than 1 GB.
For comparison, on my no-longer-factory-fresh Vista-64 system, after everything is loaded, about 1 GB is used. Windows 7 is lower than Vista, so again, W8 should be definitely less than 1 GB.
Note that even if they started a "typical workload" and it turned out to be 1.25 or 1.5 GB total, you'd still put "2 GB" on the box. Very few people buy systems with oddball amounts of RAM. (For a while, vendors were selling 4 GB boxes with 32-bit Windows, leaving about a half-gig unused.) Many consumers don't think about whether that's "too much" and they'll end up happier with the performance.