Apple hasn't defined how much bandwidth a typical video call will take, so "there is no good way to know what it will cost users," Gold added.

FaceTime over cellular "certainly [won't work out] if it raises people's bills" with carriers, Gold said.

Stories of for downloading video over a cellular connection are growing commonplace and AT&T and Verizon now offer tips and smartphone monitoring tools to guard against exceeding monthly data limits. They also send warnings to users when limits are exceeded.

Still, customers surprises could increase when FaceTime goes cellular.

One Computerworld reader who asked not to be named said she was surprised recently to get an $80 unexpected addition to her Verizon bill primarily for downloading a video attachment from a relative sent via email on her new iPhone operating over 3G. The video was less than 10 minutes long, she said, although she confessed she wasn't familiar with the terms of her new data plan or how she should monitor it.