I've bought those 100 packs of 38 special +Ps a couple of times...the price for them at the local Wal-Mart is about $34.
The Remington 100 pack of .357 magnums is about $50.
Since I have both 38 specials and .357 magnums, I'm tempted to stop by Wal-Mart this afternoon and see if I can find any of the affected packs.
This definitely could be bad, though, especially if your 38 special revolver has the cylinder bored too deeply(none of mine will chamber a .357-I've checked). With that said, though, I've been told that at least on relatively modern Smiths(1960s and newer) all of the frames and cylinders get the same heat treatment regardless of what they'll ultimately be chambered in. Thus, firing a 357 magnum in one not designed for it probably wouldn't cause a kaboom, although it certainly wouldn't be good for the gun in the long term.