I can find nothing about what the action editor does or how it works, apart from a few lines in the wiki. Is there a tutorial out there that describes this feature in detail?
Have you read both the Manual and References pages on it ? :
http://mediawiki.blender.org/index.php/Reference/Windows/Action
http://mediawiki.blender.org/index.php/Manual/The_Action_Editor
The ActionWindow gives an overview of the current Armature Keys or Relative Vertex Keys or Object keys for an Armature or Object. Each key (bone position) is a small diamond, yellow for selected keys, gray for un-selected. It presents the time, in frames, on a horizontal axis and, vertically, in as many stripes as there are bones in the armature, or Relative Vertex Keys.
It’s most commonly used with Armatures, though any other object can have an Object Action if you enable the “headless man” icon in the IPO editor.
So for an example, create an armature with at least two bones in it, put it into Pose Mode (CTR-Tab), the create some loc/rot keys at different frames for each bone… You’ll see an Action created in the Action window. Put the mouse cursr on that window and press HOME to see all the keys. You can then move the keys around in the action editor to adjust the timing of the animation.
The idea is that if you move one key in the action editor for a partiuclar bone it affects all the Ipo channels at once. You can select multiple keys in the Action editor and “scale” them, which has the effect of compressing / expanding time.
If you have further questions ask away 
Mike
I read both those pages, but found them a bit lacking (like learning about your new printer by reading the whitepapers…). Tutorials or guided examples would be nice. It seems the action editor holds a lot of functionality I have been looking for elsewhere!
EDIT:
Something of -great- particular interest… How do I make keys with sliders that represent transformations (e.g. rotation)? Something like described in this:
If the object is an armature, each bone keys is represented as a grey diamond at the pertinent frame, or yellow if the key is selected. If it showing Mesh keys, the diamonds are present where a Key is assigned a given value. This can be assigned via IPO or via the sliders in the ActionWindow.
If the “Sets IPO to be included as Action” button is enabled in the IPO header (looks like a headless man … beside the IPO type dropdown), then Object IPO types will have Action keys created for them.
As I mentioned, the main use for is when animating Armatures. When you keyframe an Armture -bone, not only does a key get written in the IPO window for each animatoin channel affected (Loc X/Y/Z , Rot X/Y/Z Scale X/YZ/),. but a correspoding
key gets wrtten in the Action editor. But only one key is written in the Action Editor per bone, while up to 10 channels could be written in the IPO editor (Loc, Rot, Scale …Rot displays 4 channels using Quat rotation).
So if you keyframe 3 bones at once in a character, say the L/R feet and the torso, you’d get 3 keys in the Action editor (one for each bone), but up to 30 channels in the IPO window. If you create say 30 keyframes for all 3 bones at different time frames … say for a walk cycle, and you disvcover the left foot for example is consistenly too slow, you could just select the “row” of keys in the action editor for the left foot and “scale” them which actually moves all the selected keys closer toegether, which will “behind the scenes” actually affect many IPO curves for you, instead of having to go in and mess with the IPO curves.
Another thing the action editor gives you is the ability to “package” and label an entire …errr Action, e.g. a “walk”, or a “hand wave”
The “Intro To Character Animation” on the Wiki in the Summer of Docs (link in my signature), shows how to do that, and then how to setup actions in the NLA editor and “mix” them together.
The action editor can also be used to control Shape keys (Facial Morphing), but typically most character rigs will have some kind of controls in the viewport using “driven keys” to indirectly control the shapes.
What functionality have you been looking for?
Mike
Those sliders are automatically created for “shape keys” (select a mesh, then on the edit buttons (F9) / Shapes panel, click on the Add Shape Key button
http://mediawiki.blender.org/index.php/Manual/Shape_Keys
For controlling rotation/location, you’ll have to setup “sliders” in the viewport, using
any object you want as described in :
http://mediawiki.blender.org/index.php/Manual/PartIX/Driven_Shape_Keys
(The difference is that instead of selecting a Shape IPO to apply the IPO driver to, you’d select an Object IPO + Loc / Rot channel).
Mike
By way of tangent, you may just have answered my prayers! It seems IPO drivers was the secret ingredient I was looking for, apparently for all of my troubles… Yet another case of digging for treasure on the strange island of Blender
There’s also PyDrivers, which are even more powerful. (Hook arbitrary Python code up as the driving element) :
http://mediawiki.blender.org/index.php/BlenderDev/PyDrivers
Mike