I've been working on the issue of getting the cache to clear after programatically adding a role to the logged in user.  I found the the code you posted above (along with a few variations of it on a few other posts). 

Response.Cookies("portalroles").Value = Nothing
Response.Cookies("portalroles").Path = "/"
Response.Cookies("portalroles").Expires = DateTime.Now.AddYears(-30)

I got excited because it works beautifull on my development laptop.  The issue is once i moved the code to the development server in my office the cache stopped clearing and the page i directed the user too now gets a "you dont have access...." message.   Its the same code.... you wouldn't have any idea why this is?  Is this a configuration thing on the server or within DNN config that would make it work one place and not the other. 

I also found some code in another post to actually "log out and log in" the user on the fly,  but this gave the same results. 

I'm hitting a wall,  all i want to do is assign a new role to the current logged in user (from my custom module) and then redirect them to a page that that role has access too.

Please help if you or anyone else can. 

I've had this same problem intermittently for a while. I've been through the forums here for help but just found that lots of us have the issue. So I decided to debug myself. Likewise for me, all works well in my dev environment, but not on QA and production. I believe I've found the issue and I've put in a bug report. I'd bet in your dev environment you run your database server on the same machine as your web server and in your live environment they are on separate machines. Take a look at http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=7070&PROJID=23. I put my workaround in there. If that doesn't work for you, after you understand the problem, you'll probably be able to come up with another.

One final note: You still need to clear the "portalroles" cookie and the user cache sometime between adding the new role and redirecting to a page that requires the new role. The following two lines of code will do it:

         DotNetNuke.Security.PortalSecurity.ClearRoles();
         DataCache.ClearUserCache(PortalId, Username);