lol, microsoft and support. The thought is almost comical.
But re: open source support, its actually a lot more prevalent than you think. The Linux Professional Institute is doing a great job and so far has certified over 100,000 junior linux administrators, and a few thousand senior administrators (which still means we get the big bucks... *cough, cough*... yea I almost choked on that one...). As was pointed out, if you need support beyond this in the realm of specific applications, there are consulting firms willing and able to do it, but as well I should further point out that open source projects do tend to have an involved community willing to help out, as well as a searchable bug database. So you can normally either quickly fix your issue on your own with community assistance, or identify it as a flaw and take steps to minimize the impact of it. Saying that, I am on the monkey make work administration side and don't need to grapple with coding or anything along those lines.
In the end, the biggest difference that I've found is that you do spend a lot more time reading documentation and learning to administer and troubleshoot the product via your own devices - which can produce issues for a busy yet small IT department who don't have hours to spare training staff.
oh and dapaterson, you forgot one "The problem lies with third party software, call them"