Let me start by asking for tolerence on a newbie question, but I've done a lot of searching and can't find the answer to my question.

On a number of occassions recently I've noticed my connection status has changed from 'Leaf' to 'Ultrapeer'. How does this come about?

That's good: it means you have a high uptime, good bandwidth, and can accept incoming connections.

In other words, your LW is set up well enough that you can be the one looking after the leafs. The process is automatic, and depends on the demand.

Leafs connect to ultrapeers, which are the backbone of the network.