There are a few reasons why the suit has been filed at this time. The company can argue that losses due to patent violations would hit it harder at times like these and force more staff cuts. That would not play well with the US government, which is seeking to stem the bleeding of jobs in every direction.
The impact of likely job cuts at Microsoft would reverberate around the tech industry because so many outfits are dependent on business from Microsoft to keep their books in the black.
The second reason why Microsoft has sued now is because it is easier to put someone out of business at this time - even buying up a company would be much cheaper. Additionally, TomTom is an ant compared to Microsoft and if Redmond did manage to extinguish its operations, it would be a nice slap in the face for a company which it tried to buy in 2006 - and failed.
It would also be able to pick up TomTom at a bargain basement price. This is classic Microsoft - never develop your own technology when you can bully someone and then buy them.
(On the other hand, if the patent suit fails, we may well have seen the last of Steve Ballmer. The challenge of trying to find a way of keeping profits where they once were in a changing software environment has not been very successfully met by the Monkey Boy and if he fails this time, he may well have to ride off into the sunset.)
A third reason why Microsoft thought it a good idea to begin a patent lawsuit is because a case like this would not run for very long; it would be an economical way of sending a warning to all the little embedded manufacturers that they could be next.
Extracting hush-hush money from other companies would then become so much easier. Controlling the embedded market would be within Microsoft's range.
And finally, suing at this time sends a message about patents - mostly FUD - without tangling with any of the big companies who are deeply into Linux - IBM and HP, to name just two. In better times, it is quite likely that bigger companies would become parties to a suit like this in order to take revenge on Microsoft. But in the current environment, that is unlikely.
But there is one thing which a corporate entity like Microsoft can never comprehend. And that is the energy of the free software community, the anger and hatred that the lawsuit has generated.
When SCO started its campaign against Linux by suing IBM, it was quite confident that things would go its way. Six years later, the company is just a shell and few people would even bother pissing on it.
Somehow I have the feeling that this time Microsoft may have bitten off more than even it can chew.