It may not be politically correct to say, but 64-bit Linux running on commodity hardware powered by Advanced Micro Devices and Intel won't smite the classic mid-range muscle. The Linux camp, of course, says it can, citing low price and an open architecture. But the tightly coupled hardware/software design and lockstep evolution of the classic 64-bit behemoths offers advantages that Linux can't - and won't - match.
Take a look at the top contenders. Sun Microsystems' Solaris server running on a Sparc processor is the company's flagship and the industry's most visible 64-bit system. IBM similarly touts its two Power4-Based eServer families, the iSeries (formerly AS/400) and the pSeries (formerly RS/6000). The iSeries runs a proprietary operating system called OS/400; the pSeries can run either Linux or AIX. IBM and Sun are aggressively improving their 64-bit platforms.
These systems share a tight integration of hardware, operating systems, drivers, and management tools, which even the closely held Apple Computer Macintosh lacks. With complete control over every stage of development, all these midrange systems are scalable and extensible. You'll pay (oh, how you'll pay), but these are bet-the-business types of servers that simply won't let you down.
That's not to say that there's no culling of the mid-range herd. The third major player in the proprietary 64-bit space is Hewlett-Packard, and its platforms' days are numbered. HP is aggressively trying to nudge its Alpha and PA-RISC customers toward its Itanium 2-based servers running either HP-UX or Linux.
By contrast to these tightly engineered Unix systems, 64-bit Linux suffers from the built-by-committee curse. It runs great, but it was designed without regard to the hardware. The idea of a single, unforked Linux kernel may be great for compatibility with common, off-the-shelf hardware but not for optimising the OS for a specific server design. So, yes, Linux is fantastic, but each distribution is a collection of distinct code modules rather than a single, carefully designed platform. You get portability and enthusiasm, but are those enough?
Because they don't have to worry about little things such as cross-platform compatibility, the IBM and Sun (and HP) teams have a tremendous advantage in designing and building their 64-bit proprietary platforms. They can describe specific technology roadmaps, deliver on specific schedules, and incorporate new technologies without waiting for public consensus or even industry standards. Furthermore, they can guarantee that their solutions will work, backing their promises with hard-core SLAs.
Linux will always be more affordable and compatible, and it will end up with the lion's share of the market. But there's no way that Linux can ever offer the tight coupling of OS and hardware provided by those purpose-built midrange alternatives. For some customers and applications, the business case for that tight coupling won't ever go away.