After much experimentation and digging around on google groups (special thanks to Ibrahim Hadad) I have finally managed to get Monodevelop 3 and F# working together nicely on Ubuntu. These were the steps I took. Your mileage may vary. :)

(Update: Knocte has suggested a couple of modifications to simplify the process. These are now reflected below.)

1) sudo apt-get install mono-complete libgdiplus git autoconf libtool

2) Install monodevelop using the script from John Ruiz’ blog:
http://blog.johnruiz.com/2012/05/installing-monodevelop-3-on-ubuntu.html

3) Get F# source and compile:
git clone git://github.com/fsharp/fsharp
cd fsharp/
./autogen.sh --prefix=/usr
make
sudo make install

4) Run monodevelop. Go to tools, add-in manager, gallery. Install F# language binding.

5) Enjoy!