As we've been discussing the TM icon lately, and I'm one of those who isn't keen on the current icon, I thought I'd do some research.
When placing a new brand into an existing market, one of the things to identify is the values presented within the brands of your competition. For icons, the overriding value to consider is that of colour.
I'm no designer, but I've grabbed together a bunch of OS X editor icons and placed them on a colour wheel:
http://allinthehead.com/assets/img/osxeditorcolours.png
As you can see, the main focus is on blues and greens. These colours are already over-used and should be avoided. Strong yellow and red have also already been claimed, although there's no strong orange in use.
The main gap on the wheel is obviously the pinks/purples and the cyan end. Cyan is tricky to get right, as it tends to either drift into blue or green, or contrasts badly against white.
Therefore, I think that TextMate should have a pink/purple icon.
drew.