I don't think the reason keeping most people from ordering the 3500mAh extended battery for the HTC EVO is a lack of compatible cases, but on the off-chance that it is, you might like to know that Seidio is working on an Innocase Snap that will accommodate the extra junk in the handset's trunk.
The image you see on the right is "for illustration purposes only" and therefore subject to change, but I don't know how else the hard case could be designed without adding even more bulk so I'm going to commit this design to my short-term memory. The upcoming case is listed as being "under development," so it's no surprise that a price and release date are both unknown.
There's no mention of a special holster that will be needed for this case so maybe it's compatible with the existing ones. Either that or Seidio doesn't want you to wear it on your belt in case you topple over from the extra weight hanging off one side of your body.
In all seriousness, I think this case looks better than the extended battery alone. The bulk seems more streamlined and a little better disguised to me.