Hi Mike
What browsers - DNN has been designed from IE 5.5 and above, but you can view in all browsers, like most sites. If you are talking about usability, then I believe that IE 5.5 & Above will do the job fine, but of course it's better in the later browsers as technology is better as it gets developed.
If you are talking about usability, then that's another question that needs to be answered.. eg.. some browsers don't like IFrames -that's when you put content from one site within another using a known function as the IFrame, so if that was an important thing as part of your development, then it could be regarded as a problem. But you don't have to use that function to run a dnn site.
If you are talking about the menu and navigation, then yes there are some browser issues, but you would have to establish who is your audience, and look for third party solutions. For example - some schools use Mac for their choses operating system and in there there used to be some problems with all the menus displaying as they should, in Mac IE 5 and some safari browsers, but there is a work around, like putting in third party menus.
There might be some issues for some browsers to use the core editor that comes with DNN - the FTB (Free Text Box) editor. It's got some special quirks of it's own that affect Firefox & Netscape users, but there is a free and commercial solution, in fact, we've got a new project with the FCKEditor - and that means you can choose and install this editor for your company instead.
These would be the main areas I see have caused some questions for people regarding compatibility, and as you can also see, DotNetNuke developers and the DotNetNuke community at large work together to find solutions.
DNN has been around for a few years now, and has a huge developer base, along with so many different variants of special requirements and to be honest, I've not spoken to many people who have felt that it's not worth pursuing, due to compatability issues. There have usually been enough people able to work together to make this a successful installation for the company, personal project, government application, client solution, non profit organisation and more.
You see, it's hard for some people to get their head around with open source. How good can it be? It's free... And yes, there some limitations, and yes there are some quirks and problems, the same as in every other application, but in my opinion, overall, I've yet to see a commercial product, offer the same value and DotNetNuke does, which is one of the main reasons I am so excited and committed to working within this space.
My interest is to provide solutions for clients as well, and you'll find here, that the many many people who contribut here, whether they are on the core team or not, simply love this application for what it can do.
Your IT guy should not have any problems with this product, but it might also help if you were to specify the areas you need to make sure are covered, so you can judge what's out there to help you move on with the project. Eg, special applications, special modules, unique purpose that is outside the normal installation or not able to be picked up and applied fairly quickly.
Your questions are valid, and I've only given you my perspective, but I'm sure that many people share the same view, or DNN wouldn't be growing at the rate it is.. and it's only going to get better.
Thanks for you post and thanks for reading a moment of my admiration for this project and the community that support it and make it what it is.
Nina Meiers