The online compiler is nice to get started quickly, but it offers you no freedom. I'm not only talking about simple freedoms like changing editor prefrences (tab indentation, different colour scheme, etc..), I'm talking about being vendor independent.

It may seem as if the mbed.org platform is open, but the truth is that every mbed program links against a closed source library. This is classic vendor lock-in and comes with the usual problems. For example, if mbed.org one day ceases to exist or starts charging money, everyone is screwed. This is sufficient reason not to use the online compiler, at least not untill the results can be relliably reproduced offline, using exclusively open source tools.

The forum shows that there is a sizable demand for an offline toolchain, and several attempts have been started.

Step 1: Get a GCC toolchain that produces M3 code:

http://www.state-machine.com/arm/Building_bare-metal_ARM_with_GNU.pdf

http://www.kunen.org/uC/gnu_tool.html

Step 2: ???

Step 3: Profit!