You can do almost exactly that in Resolver One (http://www.resolversystems.com/), which has got a Python console as well as allowing Python functions and Python syntax in the grid. It's a separate spreadsheet program, though, not an Excel plug-in.
Alternatively, there's PyXLL (http://www.pyxll.com/), which lets you write Excel user-defined functions in Python, but doesn't have a console.
There are actually two third-party plugins for Excel that let you use IronPython for scripting to some degree: DiscoveryScript at http://www.xefion.com/getting_started_diss.html and PyXLL at http://www.pyxll.com/ -- I don't use either, though, so I can't comment on how good they are.
(BTW thanks for the kind words about Resolver One -- that's my day job :-)
Alternatively, there's PyXLL (http://www.pyxll.com/), which lets you write Excel user-defined functions in Python, but doesn't have a console.
(Full disclosure: I work for Resolver Systems.)