1. You have to spend money to teach the admins how to handle the new systems, backup and make the older backups of the network 100% rolloverable for linux.

2. You have to spend money training the employees on the differences so they can use the OS effectivley and efficiently, compared to the OS they already know how to use.

3. You have to spend money on code conversion, to get it Linux ready and usable for any company software.
I agree with pooh sun tzu but why argue.
With Linux with all its versions will keep us employed. The reason for this is because with Linux as a *nix OS we are literally taking 2 steps back Linux is playing catch-up to expensive and much better os like AIX and such but like I said Linux will keep us employed. And in time Linux will catch up. Ok before I get flamed about how great os Linux I will say that I am a devote Linux fan.
Linux for ever!! (Or until something even more fun shows up)