Twitter for Mac 2.0 released with a black primary and a black 3-D style secondary icon (inside Resources folder).
January 12, 2011: Twitter for Mac 2.0.1 released with a new light blue primary icon with the previous icons removed.

January 18, 2011: Twitter
for Mac 2.0.2 released with a different blue colored 3-D style primary
icon with the world as the background along with the two previous
primary icons remaining in the Resources folder.
At this rate, it looks like January 24,
2011 will be the release date for Twitter for Mac 2.0.3 with an option
to select which icon you want to use.1
[/sarcasm]