Does a webinar always require a phone line and a computer?
No it does not. Most of the web conferencing services offer VOIP (voice over IP), which means you can send your voice through a microphone on your computer and hear the audio through your computer's speakers (sent over the internet). I like to use a telephone for the audio because it doesn't take up so much bandwidth and it doesn't cause delays with my slides showing up on the screen. Also the telephone is a comfortable piece of technology for everyone....everyone has one. Not everyone has a microphone for their computer.
Can you recommend any online resources for developing curricula for webinar format?
I do have some listed on my resource page. Another good source is the eLearning Guild http://www.elearningguild.com. Its also a good idea to participate in other webinars (regardless of the content) to just see how other people are presenting. Its a nice way to get new ideas.
I find it annoying on conference calls when participants are "announced" into the call. Does this not happen on webinars?
That's really going to depend upon your teleconference service. Our service emits tones when someone joins or drops of a call, but I believe we have the option to enable it. It is an interruption so if you can disable it, you should.
Can a participant ever advance slides or go backwards?
That depends upon the software you select. Some services do allow your participants to advance slides as needed but its a feature that needs to be enabled/disabled.