I just spent the last 20 minutes texting photos back and forth from/to my S2 and my wife's 4.
Sending multiple photos in one text from the 4 to my S2 results in the slide show you guys are talking about. A single photo to an from each phone results in exactly that, a single photo with no slideshow issues.
If I send multiple photos in a single text from the 4 I get a slideshow.
If I send multiple photos from the S2 to her 4 she sees multiple photos in the same text and not a "play" button to view the photos.
I am using Handcet to manage my SMS/MMS.