Apple's 10.3 had some 64-bitness and Tiger has more of the system running in 64bit mode.
Sun's Solaris OS has been 64bit for many years and runs in 64bit mode on Opterons.
Linux has been 64bit for many years and runs in 64bit mode on Opterons as well.
Not really - they had 64-bit versions of XP back in 2001.
64-bit OSes are a necessity for server operating systems which may well have a need for an address space greater than 32 bits. For most desktop and home uses there is not yet any great need for more than 4GB of memory, not does the larger word size yield any great improvements in the power of the systems. In fact, 64 bit chips can be less efficient and may produce more heat due to all the extra address lines used internally. However, as Intel and AMD switch over to producing all 64-bit chips we can expect the operating systems to follow eventually, but I don't think most average users will see any real improvements related to the switch to 64-bit processing itself, although of course the chips themselves will continue to increase in speed. Eventually as costs continue to fall and software continues to bloat most users probably will be using systems with more than 4GB of memory. The next MS IE will probably have a memory footprint of 1 GB all on its own (only partially kidding).
Microsoft is actually the LAST mainstream provider of operating systems to hit 64bits.
- Apple's 10.3 had some 64-bitness and Tiger has more of the system running in 64bit mode.
- Sun's Solaris OS has been 64bit for many years and runs in 64bit mode on Opterons.
- Linux has been 64bit for many years and runs in 64bit mode on Opterons as well.
My Alpha (bought surplus for $25 several years ago after it had been retired by a university) laughs at your puny list of 64-bit newcomers.