(about this placeholder)Edit
Since there has been nothing written on this page for a while, I asked for some basic information on another forum. I'll just quote what I learned from that forum, and this can be massaged into a real page of documentation over time. I won't attribute the individual various quotes, but thank you all for helping out. Remember, ANYONE can edit a Wiki, so create an account and make this thing better.
The NLA WindowEdit
It's quite easy and maybe that's why there's no specific tutorial. Let's say you want to make two actions, AC:Hit and AC:Kick. Start with posing Hit and an Action will automatically be created in the Action Editor consisting of all the Bones that use Action IPO's. That's done so return to Frame 1 which will be your default Stance of AC:Hit.
Now, in the Action Editor, click the X (delete) next to AC:Hit and the datablock menu will disappear. (If you Add New instead of deleting then it will copy selected bones to the new action and you don't always want that). If you want a new default pose for AC:Kick, then Pose it or the same stance will be used as was the default in AC:Hit. Pose and Keyframe your Kick action and name it.
Over in the NLA Editor you can now use Shift-A to add NLA-Strips of your Actions, Grab and Scale them and use the Transform Properties tab to input how they Blend.
If you select an Action with the dropdown menu, at first its name will appear in the NLA window along with its keys. To make this Action into an NLA strip, point at the Action's name in the NLA and press CKEY.
Close any open Actions by clicking the scary X in the Action Editor. If you don't do this, only this action will play. Now in the NLA editor, play the animation.
If you see any keys (diamonds) in the NLA window, instead of strips (rectangles), you're still editing an action. It's so much easier if you have both the Action and NLA windows open so you can see whether an Action is open or not. (Edited by CD38 23 Feb 2006)
With no Actions selected, both the Action Editor and the NLA Editor appear empty. Here, the NLA Editor window does list one Object called -Empty because that object is not an armature but it has some IPO curves attached.
Select an Action you've already made. Here, an Armature named Yui has bones involved in a one-frame action called -AtEase.
Convert the listed Action to an NLA Strip in the NLA Editor by pressing the CKEY with the mouse hovering over the Action to be converted. No change in the Action Editor; it is still available as an Action.
Once converted, note the changes in the NLA Editor. The Action icon appears next to the Armature's name: . This is actually a button though it does not look like it, and you can toggle it between the Action and NLA Strips icon by clicking on it: .