I know I will be installing this on my SSD as well. I'm not sure how the game will be caching previously visited areas in the same play, though I'm sure some tech website will post some SSD benchmarks for D3 soon enough.
It seems like with the use of waypoints, town portals, and smallish sized floors with multi level dungeons that an SSD would be extremely benificial. Much more so than in WoW of Battlefield 3. In the beta I was changing areas every 5-10 minutes so 4 or 5 seconds could add up real quick.
In my opinion when it comes to Diablo3 a SSD is the best upgrade for any rig that is not completely outdated.
Since you will get good fps at good resolutions and high details even if your computer is a bit weak, the only thing which holds you back or reduces your "farming-efficiency" are loading times. Even though Diablo 3 does not seem to require as many new games and as much area-changing as its predecessors, it might still be important.
Actually it will give you a very noticable performance improvement in Diablo 3 at the moment.
Currently there is a bug/design oversignt with Diablo 3 where it dosent pre load assets on level load, so when you first start playing every time you use a skill, see a new mob etc it will cause the game to stutter very badly as it's trying to load the assets from the HDD on the fly, unfortunatly most HDD's rnt fast enough to do this at the moment.
I had this issue so copied the game over to my revodrive, bam, instant fix & smooth as silk.