I've decided to use Flash to provide the UI for my persistent browser based game Perenthia. I went back and forth between Flash and AJAX and even looked into Silverlight a little and Flash just has the maturity needed for a good PBBG interface.  I originally had the main game UI written using AJAX but found it to be a little cumbersome when a lot of activity was occuring on the back end, just too much traffic generated for one user. I looked into Silverlight a little but I am going to wait until the 1.1 version is released so I can program in C# on the backend. Flash seems to be able to provide me with what I need and I wrote custom ASHX handlers on the ASP.NET side to handle commands from the Flash UI. The UI is basically just an advanced MUD client, in that the primary output is text based. However, with Flash I will be able to provide a better map and add some additional graphical features later on down the road. Since the command handler is a custom ASHX handler in .NET I could really allow any type of client to connect, as long as that client can send XML as an HTTP POST and receive the and parse the XML response from the page.

I will post a screen shot of the UI in the next day or so.