There are really two factors here (well three if you want to include your risk tolerance and savviness about locating yourself and your business in disparate parts of the world).
Primarily, as mentioned earlier, it really varies from company to company. Don't assume that old-school companies are easier targets (witness Disney for example). Some companies are TM-hawks, while others don't really care or don't notice.
Which leads to the other factor - it's fairly easy to fly below the radar and skim off enough sales/revenue to make a TM name worthwhile.
And remember, on the one hand there is no actual Internet Jail where they can put you for violating a TM; on the other hand, if you choose to build and invest in a TM site, you could lose it or at the least face the onerous process of a WIPO dispute, C&D letters, litigation, and the like.
peace - gregs.