I need to build a very simple administration interface for maintaining website content. Not fancy enough to justify Umbraco or a more heavy duty CMS.
Basically just a couple of formatted text areas (a bit like the one I'm using now) and the ability to upload images.
I've used FCK in the past, but is there an improvement on this for ASP.NET these days?
Take a look at the source code of this book: ASP.NET 3.5 CMS Development.
The book itself is about writing a simple CMS system with ASP.NET. Take a look here for a review.
I spent a bit of time researching ASP.NET CMS options recently. N2 has gotten some great reviews and sounds like it would work well for you since it's pretty lightweight. I haven't used it yet, but plan to give it a shot when I start my next project.
If you just want to convert a textbox into a rich editor but don't want a whole CMS, you could use a lightweight jQuery plugin...
http://www.queness.com/post/212/10-jquery-and-non-jquery-javascript-rich-text-editors