i get the feeling lots of people confuse the two....

note the number of vids on youtube teaching "bellyrolls" that are actually undulations....

I agree, tho we could call it an undulation of the abdomen.... ;-) maybe there's a translation issue here.
I teach undulations always & belly rolls hardly ever. Many students don't want to learn if I tell them it will make their tummies pouch out.

Many dancers emphasize the full body undulation (or camel or roll up/down) with a belly roll layered on top.

"I think the two are more connected than we might think of them as."

I agree... although if you stricktly look at muscle movement... (via suhaila method) it's upper back upper abs, lower abs( rolling up to down) . well for most people that lower abs part is really the middle. I think for a true 'belly roll' persay you have to engage the lower low abs.

Other than that I think one is an exageration of the other to get a more pronounced effect, almost like a cheat to get a similar effect. But you do have to use the most of the same muscles.

my two cents, which aren't worth much I guess considering I can' really do a belly roll... more like blub blub blub.... insort of a nausating sea saw effect. =p