Thanks to a conversation moments ago with Eileen I now know the answer!

I rebuilt my production laptop yesterday and as I was in meetings much of the day I didn't get to access the corporate network for very long at all on it - I didn't have wireless access during the meetings as we have to connect via wired networking for a few moments to get the certificate required for access to our wireless infrastructure - it's automatically provisioned once we plug in. It turns out that my machine didn't get chance to download the offline replica of the corporate address book (which is huge ~ 90,000 people) or the calenders of those I regularly browse.

As I'm working @ home this afternoon (following a meeting in London earlier) my poor old machine is struggling to reconcile it's Outlook data hence it's rather busy.