I went to a talk by DHH once, and he said that they consciously try to pick fights with the big boys, and hope they respond. This accomplishes two things:

1) They get a lot of attention, and thus brand recognition

2) They move up into a league of players, where they really don't belong if you look at their business in terms of turnover, employees, etc. Basically its 37signals vs. Don Norman, 37signals vs. Microsoft etc. As if they were equal.

This is an example of that. And its brilliant marketing.