I think the first thing is to do as Edhy suggested and have some meta data driven way to manage this, likely via a table(s) to manage the association of roles/groups/users to forms. You could also use a configuration file or a web service or ... Tables are easiest.
Then you'd use BOs to access this data. Then you'd need to manage some UI elements. Edhy suggested an Ingragistics tool, but there are many choices, including standard .NET or SF tools. Here are some suggestions:
1. Create menu with all items shown, then hide the ones that the user doesn't have access to. This might seem the easiest, but can be a bit challenging as you have to do lots of looks up.
2. Load a combo in a menu/toolstrip with the list of forms they can open. This is easy, but not very friendly.
3. Use a navigation panel on the left of your main form (assuming MDI here), and load either a SF Listbox or a SF ListView. This is even easier, as you can just use the standard SF list population methods to load the list of forms for a user.
4. Use a SF ThemedLinkMenu in the panel on the left. This looks slick and has some nice UI elements. You'd have to manage this more manually. I.e. get the data, then create the ThemedLinkMenuItems. However, there are samples of how to do this in the StrataFlix sample. In that case, they are using the ThemedLinkMenu for actions (like edit, search, delete), but the concept is the same. Also, I believe they sub class the ThemedLinkMenuItems, so that might be something you'd do also, such that each ThemedLinkMenuItem would know how to actually open the form.
5. Create a switchboard form, that has a list of forms they can use. Here you could use just about anything, including the listbox, listview, a tree control, create a list of link labels. This can be nice to provide context for occasional users or beginners, but can get old for experienced users, who'd appreciate faster navigation.
Then there are all of the other third party options, like Edhy suggested. Many of these will have Office 2007 like ribbon options or Outlook like options.
I hope this sparks some ideas and/or conversation on the topic for you!