As the IT Director in a company of about 50 desktops/laptops I did exactly that- installed Ubuntu on a couple of managers' (not computer savvy users) computers. ~2 years ago I installed Ubuntu 8.04 on their computers, converting them from XP. Haven't heard a complaint or had any major issue. Their main apps are web/email/documents/spreadsheets and a terminal program into the billing system.
Not only have they not had any problems, they had little to no trouble adjusting. They interact with all of our external suppliers/vendors/salespeople in sending and receiving Word and Excel using Open Office. Almost every other computer in this company has had malware/virus/rootkit problems of some sort (all XP machines).
So not only have I saved the company money on Windows/Office licensing, I also reduced downtime from the potential of malware. As computers are replaced here I'm replacing them with Ubuntu everywhere possible.
Don't be afraid and maybe give Ubuntu a try (or other nix distro) in your organization(s) (and before you flame me, yes I understand that many companies use Windows only software, so you are excluded, there are many organizations that only need web/email/docs/speadsheets/pdfs).