Yes, from when I looked into it the bottleneck seemed to be the SMSC (short message service center), which receives the message then sends it out to users when they are available. Apparently, SMSC's often became overloaded with the popularity of SMS. You're right, SMS I believe takes place on a separate mobile network rather than the open Internet, which is why it's subject to the fees and different behavior. I'm at a point where I'm torn between writing SMS technology off in favor of email, or considering it for certain things, because we're still a long way away from everyone comfortably accessing their email via mobile.