I've put a good number of Linux servers into SMB over the past year.
They won't do it on the desktop - they've already bought an operating system they know how to use, and they need the application support that Windows provides. They need AutoCAD, they need Sage Accounts, they need it supported by the application vendor. END OF STORY. Don't do the long-playing-record thing with them - you'll just turn them off. As far as the server goes though, to them it's a black box that sits in the corner. They feed it a tape every night. They don't care how it does what it does, as long as it does it.
Let's face it, most SMB requirements aren't that complex. Samba and Sendmail/Dovecot/Squirrelmail will deliver everything that 99% of them need, and it's all free. The backup software is free too. You don't need any CALs to access it. That, my friends, is competitive advantage.
Even on sites where they are Windows through-and-through, I've managed to find a place for Linux servers. e-Mail is an easy target because Microsoft's implementation of SMTP is terrible, and wide-open to abuse. Put a linux box in front of it, and close it all down. You can also do a bit of spamassassin and greylisting while you're at it. Did I mention that it all free?
Database servers are also an easy target, although it does depend upon what the front-end application vendor will support. Ask the question of them. They may say "No", but by asking the question regarding MySQL or Postgres can make them think about competitive advantage for their own front-end product. At the end of the day, and ODBC or JDBC connector should go some way to making the choice of back-end irrelevant, and if the front-end software vendor can save their customer some money by deploying against an open-source database, they're one-up against their competition.