I was able to adjust the Teflon block, behind the extruder, on the rear guide bar, up so that it snugged up to the bar.  After that, I noticed that the extruder assembly still had some vertical play, so I shimmed under the extruder stepper motor with three layers of business card stock.  Not sure whether the paper will get too hot, but it is right under the fan.

Yeah.  At first I did what others have suggested, which is to try to close up the gap by moving or twisting the teflon block.  That made me a little uneasy, however, as I'd still be balancing reduced wiggle for a little more change of having the carriage bind on debris.  It's kind of like the fixed jam nut solution (versus spring-loaded preload) for reducing z-axis wobble and carries a similar risk.  I prefer a spring loaded solution because both blocks should never lose contact with a sufficient spring, and if something got caught in-between, the spring could still give to pass over it.  Also, the play doesn't increase when the blocks wear.

I was able to adjust the Teflon block, behind the extruder, on the rear guide bar, up so that it snugged up to the bar.  After that, I noticed that the extruder assembly still had some vertical play, so I shimmed under the extruder stepper motor with three layers of business card stock.  Not sure whether the paper will get too hot, but it is right under the fan.

We used to use some cardboard as a fan deflector at one point, in this very same spot. Perhaps it will work here to help keep things in line.

Good idea the spring looks like fairly thick wire, how much spring load is there? more than a couple of ounces to make the arm deflect? Also does this make any noticeable increase in friction on the rod?

The image is kind of deceiving.  The spring is made of very fine wire.  I originally stopped the bouncing using 5 quarters worth of weight (about 0.7 oz of force) and picked a spring with about the same force.  I don't think that's enough added friction to be a problem.

I moves the bottom teflon retainer block on the x-axis carriage to a spring loaded arm.  Using it, the head no longer wobbles when I'm printing near the back of the print bed, which occurred previously due to force from the cable assembly. 

This isn't the same as the z-axis wobble, which is a separate issue I have not seen yet.

As an aside, I want to thank everyone at Solidoodle for an excellent product and appreciate their efforts to continually improve their design.  Sometimes folks seem to forget this is an emerging technology, and some noodling of the device should be expected.  Here's my contribution to help make Solidoodle even better.