What kind of projects did you work on in the past? Do you want to keep the same types of projects? Meaning, if they're database oriented, then I'd suggest looking at ADO.NET (assuming you want to pick up .NET) since it's changed completely since ADO 2.x. The whole concept is different and it takes some practice to get used to the new objects.
Of course, just how forms work has also changed. You'll have to scrap any notion of global variables and an auto-created instance of a form (like in VB6 and earlier). You can have static properties which act like globals, and your default Windows app gives you one "free" form that you don't have to create yourself, but there's no more "frmNewForm.Show()" without creating it first.
Then there's error handling, debugging, the new IDE and the new files (the project files, resource files, source files, etc.). That's a lot to absorb and nothing mentioned relies on one particular language - this is just the framework and VS.
But, the first time you fill a ComboBox with an object that has distinct properties (not just a string and an optional int in the ItemData), you'll be tickled pink.
As for the market, I haven't really looked around that much. I would guess that now that VB.NET has been out for a year and upgraded to .NET 1.1, some smaller to mid sized companies might start converting over to .NET where they can. But as for which language to go with, I'd guess it depends on the company. Certainly either VB.NET or C# could do the same task in the same time (given the knowledge of the programmer). There really isn't that much different other than the syntax. It's not like comparing VB6 to VC++ 6 for instance.
-nerseus