I didn't mean it as a demo - I did it to get some work done. But now I have a wiki page that demonstrates using Sage to do symbolic mathematics in a WorkingWiki page.
It also demonstrates how awesome Sage is - in a remarkably short few bursts of code I've managed to create differential equation objects that can evaluate and plot themselves, have Sage do the timescale separation to construct the MacArthur-Levins competition model from a resource-uptake model, and use Sage's differentiation chops to construct the adaptive dynamics of the Mac-Lev model from the model itself. That's right - it generates the adaptive dynamics from the population dynamics, not me!
It then confirms that evolution reduces competition (in the one case I tried), not increases it.
Here's the link, by the way: http://lalashan.mcmaster.ca/theobio/worden/index.php/Selection_Gradients...