There is merit in this. Rumsfeld will be targetted by the investigations. The President would have had to invoke Executive Privilege to shield him from testifying and that would have perpetuated the lightning rod for 2008.
By resigning he can make himself available to testify and be done with it. And if this explanation is explicitly made, it makes perfect sense.
Yes, that is also how I understand this.
A lot of posters ask why didn't he resign before the election. The reason is of course that the President didn't want him to leave. This is the first effect of the election. Rumsfeldt knew that he would not be able to carry on with his job, since he would be a constant target of investigations, and be forced to testify in end less hearings. So like the man he is he resigned so that the President would be able to nominate someone else and get that person approved before the Senate has a Democratic majority.
As I see it Rumsfeldt didn't resign (if it is true that he has resigned) for the good of the GOP or for his own sake but to make sure that the Defence Forces would not be hurt by the political game that Democrats inevitably will start now.