Getting the iPython interactive shell working properly under 10.6 is non-trivial.
The problem is that you need to install the Gnu version of readline so that iPython can pick it up.
You might hope that easy_install would work but the readline version on pypi won't compile.
While the source package from the FSF does compile iPython doesn't see it and still complains of no readline.
Macports might work but it has an incredibly long list of dependencies you don't really need for iPython as it thinks it's part of the entire scypi kit, rather than a separate package.
Fink will install it fine. The only large dependency is that it will install a whole new Python 2.6 - but that's not such a bad thing.
You will need to use the unstable branch but just install readline first then iPython and all is well.