The "patcher" in a traditional MMO generally, in some shape or form, requires an account that has administrative privleges. This is solved in a variety of ways; for example, the new version of the Station Launcher installs a service, which allows it to apply patches using the Local System account. We are taking a different route, however.
A common approach to loading game resources is to have a "Virtual File System." Most games do this, at some level, just to manage resources. Resources could be on a CD-ROM, could be downloaded from the server, and so forth. The concept is fairly straightforward -- you have a path like: "/themes/systemFaction/systemFaction.jpg." Under the covers, the system takes the specified path and maps it to "C:\ProgramData\BattleBazaar.net\Iron\themes\systemFaction\systemFaction.jpg." You don't have to worry about where the ProgramData folder is located in the code, just one spot has to ask the operating system for it.
What we find is that most players spend most of their time in a small subset of the game world, with most MMO's. Why patch 30 zones the player doesn't care about, to patch the 3 in which they are currently playing? Now there's something -- obviously -- to maximize the use of zones. Large, outdoor zones have to be constructed with a target level, and as expansions come out, etc. people are going to move outside that target level. Using a skills centric system, like we are, doesn't change that -- if you have a large, outdoor zone the difficulty level can't really be scaled.
A strategy that helps with keeping the game footprint small, then, would be to download just what you need as you go along. This increases load time a little, but you can cache the data and perform just a quick check when returning to the zone. This removes the need to patch all the various files that go into making a zone, such as the themed textures and so forth. Meanwhile, it means when you add new content, you just add the new content and immediately start referencing it. That rules out dial up, but dial up is not a huge market segment anymore.
Now, extending this to the application itself -- if you trust the application using a digital signature, then the assembly can be read from the cache just as easily as from program files or some other, patch-friendly, location. By trusting the application using a digital signature, you can give the application the level of access that it needs for things like OpenGL calls, but at the same time, you can load it cache-friendly from the web.
The way it works is that you mark some resources "retain," and they are always retained. You allow the user to set a minimum and maximum cache size, potentially for multiple games. Then once they download the client, everything routes through the VFS and so the assemblies are brought down automatically, and trusted as necessary via the signatures. This is similar to how Final Fantasy XI operates, for example.