<p>As a long time World of Warcraft player, and a passionate developer I have decided that I would like to combine the two and set about developing some addins.  Not only to improve my gameplay experience but as a great opportunity to learn something new.</p>

<p>Does anyone have any advice on how to go about starting out?</p>

<p>Is there an IDE one can use?  How does one go about testing?  Are there any ready made libraries available?  Or would I get a better learning experience by ignoring the libraries and building from scratch? How do I oneshot Hogger?</p>

<p>Would love to hear your advice, experiences and views.</p>

This article
explains how to start pretty well.
Your first bookmark is possibly the US Interface Forum, especially the Stickies for that:
http://us.battle.net/wow/en/forum/1011693/
Then, grab some simple addons to learn how XML and LUA interacts. The
WoWWiki HOWTO List
is a good point here as well.
One important thing to keep in mind: World of Warcraft is available in many languages. If you have a EU Account, you got an excellent testing bed by simply downloading the language Packs for Spanish, German and French. If you're an US Guy, check if you can get the Latin America version. That way, you can test it against another language version.
Once you made 1 or 2 really small and simple addons just to learn how to use it, have a look at the various frameworks.
WowAce
is a popular one, but there are others.
Just keep one thing in mind: Making an Addon is work. Maintaining one is even more work. With each new Patch, there may be breaking changes, and the next Addon will surely cause a big Exodus of Addons, just like Patch 2.0.1 did.