My point is, unless I'm missing something big, the 2.1 update is a patch that won't run by itself, so I would need some way to run it, as there is (as many have complained about, including me) no Mac patch.
1) Install Wine
2) Install VVVVVV 2.0 using Wine.
3) Find the directory Wine places the C:\ drive (I don't know where it is on a mac).
4) Place the patch files in that directory.
5) ...
6) Profit!
Oops I should have been more clear. It seems that I would have to re-buy VVVVVV to get 2.0...
Hmm... 
Where did you buy it?
If you don't have access to the Windows build, Terry is probably happy to help.
The "patch" is just an .exe, replace the VVVVVV 2.0 .exe that you get and you have 2.1 installed.
So wait, the solution has been under my nose the whole time and I can just download the 2.1 .exe and run it under Wine? 
As long as you have a 2.0 windows install.
And so, we have my problem. I only have the Mac install, not the Windows install.
You can do it with a Mac install too, as you have the data directory (the only unavailable thing here).
VVVVVV need some libraries to work, all publicly accessible on the web (the data files however aren't, obviously).
You should need these:
[632][fiq@fiq /win_drive/Program_x86/Steam/steamapps/common/vvvvvv]$ ls
data/ libpng12-0.dll* libvorbisfile-3.dll* readme.txt* SDL_image.dll* stdout.txt* zlib1.dll*
libogg-0.dll* libvorbis-0.dll* mikmod.dll* SDL.dll* SDL_mixer.dll* VVVVVV.exe*
Create a directory, name it whatever you want, "vvvvvv" or something.
Copy (or move, if you want to get rid of your mac install - not recommended...) your data directory inside the newly created folder like this: vvvvvv/data
Download the DLL files in the list above, should be easy to find by googling on them.
Place the VVVVVV 2.1 .exe in the folder
Run the game.
EDIT: The Windows version of VVVVVV is only available in 32-bit IIRC. Therefore, make sure you have the 32-bit version of the libraries.
Windows version of VVVVVV store levels and saves in the user's "my documents" folder like this:
<My Documents>/VVVVVV/(levelname)
<My Documents>/VVVVVV/Saves/(levelname/tsave/qsave/unlock).vvv
So if you want to move your levels and saves, place them in this format. In linux, they're stored the "opposite" way, a .vvvvvv directory with saves, and a directory inside named "levels" where the levels go. I don't know how they're stored in Mac, but make sure that you swap directory structure if necessary.
You might also run into an issue regarding a white screen when VVVVVV start up (screen is white, but game is responsive). If you stumble upon this, refer to the topic in the "Tech Support" forum for my solution.
If you find this way too complicated, just mail Terry and he'll help you for sure (if you have bought the game, obviously

)