@Robert, AC and Greg (In that order)
Linux costs, it is not free (in the sense of beer). If you want support, you need to pay. You many need to pay for licenses (depends). Whilst there may well be some savings in the initial purchase, you have to look at the whole picture.
Linux costs money to keep it clean as well. The OS is not magical, it needs good admins and they cost. Linux does suffer from attacks and flaws, although ones intended for Windows will not affected (obviously) and their number is reduced. This is not an excuse to run Linux without firewalls and other safeguards (well, not unless you are extremely feeble minded). As Linux gains in popularity, so will the attacks.
Printers are not readily supported on Linux. The £200 that I would need to spend to get a new printer is one of the things preventing me from switching. It's that kind of cost the people often forget about.
.Net does not compile reliably on Linux, nor does pre-compiled code run reliably. Mono has not implemented enough of the API (yes, I have tired it) to be viable. WINE is still not ready (by their own admission). Legacy systems (e.g. MS Office integrations) cannot be migrated without significant expense in terms of development, infrastructure and re-training. Many web-based business systems demand IE6 and ActiveX support - not going to happen on Linux in a reliable way.
I run Linux in virtualisation at home, my laptop runs Linux, my second box at work is a dual boot into Linux, my g/f wants me to install Linux on her laptop once her project finishes, I have another Laptop I want to beat with the Linux stick. I like Linux, I really do. But I am not so besotted that I am going to let the infatuation run away with my head.
I cannot wait for the day we get a major customer who wants to go Linux, run OpenOffice etc. It will be fantastic (I've even gone as far as to prove to our architects that I can run our server stack on Linux with ease); but I will be waiting a very long time. Until then is it MS all the way simply because that is what our customers use (SharePoint? It's getting *massive* adoption) and is thus the business standard. You must also remember that corporations move at a speed that makes glaciers seem fast.
This is not the year of Linux, come see me again in 5 years; then maybe we can see if the Penguin has gotten anywhere.