I'm watching a nice video here about making faux 3D text. I follow the tutorial, and it says to interpolate 50 times between the two text objects. Okay, fine. I understand that.

First time I went to do it, a "python" installer appeared to install something, but then interpolation failed. Every subsequent time I've tried it fails, and I have no idea what to do.

Anyone ever seen this? I'm running the latest version, .47.

It's probably not installing anything, just letting you know it's working on it.

Have you converted text to path before calling interpolation?

Yes. It was converted to a path at the beginning. What's not so clear from the tutorial is why its necessary to break the object apart after converting it. I find I have to ctrl-click each letter to individually select it, whereas the default selection is combined. (The tutorial uses a script font that needs the inside loops turned back into white, so that's perhaps an issue.)

After converting it and breaking it apart, and recombining it, (as per the tutorial,) I duplicate and move the copy, select the original then the duplicate, and interpolate it via extensions/generate from path. It just doesn't do anything more than flash a text box and disappear.

there are several topics here on interpolation problems. To start with, check to make sure that your interpolated drawing is not actually somewhere else on your canvas than where you expect it. Do this by clicking on the show drawing button on the view part of the toolbar.
If the elements are there, but moved unexpectedly, your problem is that your original had transform attributes that Inkscape is having trouble keeping straight. You can remove these attributes in various ways, but probably the most definite is to go into the xml file and edit it.
You can check you preferences to see if your transform attributes are set to be stored optimized. This should also work but you might have to joggle the objects to get their attributes optimized after you change this.

After creating the Text
Convert it to a path >Path >Object to path (Shift+Ctrl+C)
>The notification display (bottom center of the inkscape interface) should read out "Group of X objects ..." X being the amount of letters in your original text

Once converted to a path, Inkscape automatically places the converted text into a single Group, this needs to be Ungrouped, >Object >Ungroup (Shift+Ctrl+G)
>The notification display should now read out "X objects of type Path, ..." This confirms the removal of the Group attribute from your paths

Combine the ungrouped path objects using >Path >Combine (Ctrl+K)
>The notification display should read out "Path (XX nodes) in layer..." This confirms the original text is now One Path within Inkscape. This is what is required by the extension you are trying to use... the problem you may have had is that the text was grouped still and not combined, combined paths are aka compound paths.

Duplicate (Ctrl+D), You may want to Lower >Object >Lower to Bottom (End key) it in the Z-Order, then reposition it to your desired 3D placement

Now Select BOTH paths (interpolate works based on what Path is Selected First, Shift+Left Click is a good way of performing multiple selections.
>The notification display should read out "2 objects of type Path..."
Once both paths are selected now you can use the extension you are attempting
>Extensions >Generate from Path > Interpolate...

http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Extensions-GenerateFromPath.html#Extensions-Interpolate