1. Unsupported dual-mode graphics card.
2. Inability to use multiple monitors.
3. Some ports being unusuable.
4. Audio glitches.
5. Hand patching required to keep your laptop stable.
6. Poor support for X11 even with simple Intel integrated graphics.
And in all cases you will simply get atrocious battery life compared to the battery life available from the same system via windows.
People will tell you, "Oh I don't have a problem with my T61 or my T520," but their definition of "no trouble" is typically so riddled with the phrase, "Oh that doesn't work so I don't do it" that most laptops are reduced to underpowered desktops or doorstops.
Currently, there are no competitively priced linux laptops. This stems from a mix of vendor apathy and poor laptop support on Linux. If you would like to build a linux experience comparable to a mac osx or windows laptop with free software, it is currently either so expensive (Lenovo W520) or so difficult (T520) that it's almost certainly not worth your time.
You're better off using a competent VM solution to run Linux under Mac OS X and be able to enjoy a fully spectrum of modern hardware and battery life while still getting to use Linux (presumably for your development).
Note: And I sort of promised myself I wouldn't post here anymore, but on this one issue I hope this is the warning to someone that I personally never got. Don't try and make a linux laptop. It is a waste of your time and money.