jk... sort of.
I used to own a Pinnacle PCTV Togo and Hava Platinum. Both boxes supported an unecrypted RTSP feed for local multicasting of Live Television. However, they would shut off local network viewing if someone connected over the internet.
Sling on the other hand uses SlingStream 2.0. It is a proprietary encrypted VC-1 or H.264 stream depending on the attached client. The other caveat is that the Slingbox will dynamicaly optimize the output resolution based on the statistics of the received and dropped packets.
Now, don't get me wrong. I'm with you, I'd like to see this feature implemented. However, one of the reasons I stuck with the Slingbox is that the picture quality was far superior over the multicasting alternatives.
Unfortuantely, being a long time owner of Sling products I've noticed that they have extremely lose time table for implementing software. Take the iPhone client for example, I remember a few months after the orignal was released that Sling announced that they were working on a client for the iPhone, Then about a year later the 3G came out, and as far as I knew the software wasn't even submitted for review. When they finally submitted the app to the Apple, it took a while for Apple to deny them -- apparently, due to AT&T's not wanting it to bog down their network (despite it already running on other mobile clients). Then the iPhone 3.0 OS came out and a few months later the app was finally released on the iTunes store. Then almost a year passed from that point and now you can use Slingbox Player for iPhone over 3G.
There is also quite a bit of broken promises. For example, "Clip+Sling" this was announced in 2007 at CES where we would be able record content through our Slingplayers and then share them with our friends using the web player on www.sling.com. It has been three years now and not one mention has surfaced since. Also, there are a lot of mobile platforms which Sling has yet to create a client and I'm not just talking about the iPad since that was only released two months ago. Like for example the Nokia 5800, N97, amd N97 mini.
Now, I'm not trying to get your down or anything but don't hold your breath. If multiple viewing is a deal breaker for you and you have the chance to return the Slingbox, then I would recommend that you consider looking at http://www.monsoonmultimedia.com They have products which will rebroadcast live TV and can be viewed by multiple computers within the local network.
Technically they don't have an iPad app out yet. The one they have in the currently in the iTunes App Store is a standard iPhone client (non-universal). I think right now, their Live3G web portal is probably the best your gonna get but even their knowledge bases says, "Currently, EyeTV 3.3.3 software does not fully support the iPad."
You are looking at a bigger issue more than just Sling. Remember when you listen/watch a major sports game and they use the tag "this broadcast is for the exclusive use........." Sling is very careful on only letting one viewer at a time to protect the use of the product. There was a time after Sling first came out that many cable/sat companies were upset and trying to figure out how to make Sling unusable. You can more or less thank Hollywood for this law....