I must thank two of my colleagues for putting me onto this one,( thanks uBrent , thanks uJP!) - Its a free 3D Ivy Generator. It's a standalone computer programme that creates virtual Ivy for you at the click of a button, (well maybe 4 or so clicks). It's really easy to use, first download it here, for free! On this website you will see that this programme is the work of Thomas Luft, and it's available for Windows, Mac OSX, and Linux. Did I mention it's free? Once you extract the zip file all you have to do is run IvyGenerator.exe. No installation necessary - sweet! There is a readme file - you only have to read the first few sentences to figure out how to use it - it's that easy.
Basically, you import a 3d mesh into the programme (in .obj format), double-click where you want the Ivy to grow from. Then click "grow". The Ivy (stem) grows beautifully all by itself - you can stop it at any point or when it looks good. Then click "birth" to create the rest of the geometry (leaves). All that is left to do is export your Ivy (in .obj format), and your done! Well, you might need to update the materials, to suit your 3d package. (free leaf and bark textures are included with the program and there are more available for download on Thomas' site).
It's a great little app, (although a lot of thought went into making it) and I will definitely be using it in future for my architectural renderings. (I ran the programme on Windows XP 64, and rendered with 3DS Max/V-Ray, it works on Win 32 systems too.)
I made a video showing how the program works: