Hi Dave,
Make sure you are calling Page.EnableViewstate = true; in your page load or init as it is disabled by default to reduce page size where possible.
If that does not solve it, then check if AutoEventWireup is false. I think it is in my hello world examples as I generally wire up events by hand out of habit and preference.
It will probably be best to wrap this functionality in an UpdatePanel else the postback can have unexpected consequences in other features that may live on the same page. If you are working with the source code you can find a good example in mojoPortal.Features.UI/Blog/BlogModule.ascx as I'm using the ASP.NET calendar for navigation in the blog also inside an UpdatePanel.
Hope it helps,
Joe