The discussion comparing Linux/{Intel/Sun/HP/IBM/Mac} Sun/Solaris may
be interesting, but it boils down to what you need in a server and who
delivers it.  There can be and are mission critical applications being
run on Linux machines every day.  We have precedence of this happening
on a wide spread basis.  Depending upon what you wish to run, and who
supports the desired software and hardware, you will have to make your
choice between Linux or some other "commercial-grade" UNIX.

Remember, the commercial UNIX'es have 20+ years of reputation over
Linux, an infant to the POSIX OS world.  UNIX is and will be the
champion of many a Sysadmin for many years to come.

The Intel architecture itself was never intended for servers in the
first place.  It was a fairly recent phenomenon, relatively speaking,
to see Intel servers dominate the business networks.  We largely have
Microsoft to blame/praise for this in making their Windows operating
systems accessible at a low-cost entry.  Investing in a $5000 Intel
server made and continues to make accountants much happier than
investing in a $50,000 server.  Still, the Intel platform can perform
well under many applications.  

The term "mission critical" itself is misleading.  Factory
device/machine controllers are far different mission critical
operations than RDBMS transactions or packet routing.

The versatility of Linux in that it can operate well in any of these
jobs makes it quite attractive.  Especially when you consider it's
current state of portability.  Still, if your production line vendor
only supports Sun device drivers and management software, guess what
you'll be using.

Would I use Linux for "mission critical" applications?  Certainly.
I've worked with multiple business that have successfully deployed
very dependable Linux environments.  Is it the right solution for
every problem domain?  Absolutely not.  Why?

    "When you have a hammer, everything looks like a nail."

--
Chad Walstrom <chewie at wookimus.net>                 | a.k.a. ^chewie
http://www.wookimus.net/                            | s.k.a. gunnarr
Key fingerprint = B4AB D627 9CBD 687E 7A31  1950 0CC7 0B18 206C 5AFD