I'm just starting on the Ruby ladder, but wondered whether it would be
realistic to set a goal to develop live multi-user applications such as
this Flash-based one:
http://web.okaygo.co.uk/apps/letters/flashcom/index_il.htm but using
Ruby, Rails and AJAX functionality?

It's clear that Ruby's developers are still finding out exactly what's
possible, so I'd be interested to hear all your thoughts!