Linux in the enterprise comes in many forms. The initial influx for use as web servers and then for other commodity uses like file storage has proven the ability of Linux to be robust and reliable. Since then it has picked up as the platform of choice for scientific and research platforms. Being able to develop on the platform and then rely on it to keep running and cut the costs of licensing has also proven to be good for enterprises.
Now people are starting to use it more for office apps and business applications and this is making more opertunity for VARS and szupport providers.
Given the number of different distributions available and the combinations and permutations of available software the first oppertunity is to help clients to wade through this and give good guidence making the decisions to use the right one for the right job.
Then you have all of the same jobs to do as there is with Windows. Switching to Linux will not change much other then Cost. You still have to patch the OS when there are patches out.
One thing you will not have to do is sell them new hardware until the systems actually wear out. Yes everyone likes to have the newest and fastest but with linux you can run the newest kernel and applications but you can mix and match to the performance of the hardware. For instance I have an older compter (Pentium 266.) and I put the newest version of Puppy Linux on it. It runs just as fast as my newest system with Windows or my second newest with RedHat Enterprise.
Linux can do all the things Windows, Mac and Unix can do and do it cheaper. Vars will still have the same oppertunities as before and users will have the tools they require just like before but it will all be more economical.