Reason for the change is simple, BigPond is now making Second Life a metered experience. And I tend to spend a fair amount of time connected to SL for one reason or another.
I've been procrastinating a bit this evening (surprise, surprise) and I decided to check my usage for the past few months. In August I had 21gig normal usage and 25gig of unmetered usage. Definitely justifies the upgrade to a 50gig plan. But every other month I haven't been that much over 25gig, even if you count the unmetered usage. Also I've found that more and more of my work has been outside of SL. I'm helping write these papers with the Uni and starting to look closer at offline solutions for clients.