Why isn't this Scala implicit conversion working?

I posted this on SO but was hoping there's a Scala guru on this list
who can help me get a better understanding of what's going on:

http://stackoverflow.com/questions/8045594/why-isnt-this-scala-implicit-...

In particular, how do you make my original attempt to define a more
generally applicable implicit (instead of the two
less-generally-applicable implicits that are in the currently accepted
answer) work? Thanks in advance for any help.