Well they haven't said it's non-removable. And if you look at
Leaked Droid 4 Specs shows a "key" to open the back cover. Which may get to a removable battery. Or maybe just to get to the SD an SIM card.
If there is a non-removable battery, wouldn't it have been easier to put the SIM and SD card on the side with a cover over it? That's how my old Env2 worked the SD card. Never a problem to get to or just leave as is, since it didn't poke out and you couldn't get to it unless you slide off the cover of the SD slot (the cover was attached to the phone so it was never lost either).
But yes, it's going to be more heavy. I agree the D1 is heavy. Anytime I pick up anyone elses phone, I can't believe how light their phone is.