It has been about 10 months since Salesforce acquried Manymoon - a simple task management platform very popular on Google Apps Marketplace.
What have they done with it since? Well, for one, they just launched arguably the cutest ad campaign in tech, telling a story of a the 3rd graders running for class president.
What does Do.com mean for Salesforce? Most probably a way to get the new online teams onto Salesforce platform while they are young..
Do.com has a nice simple HTML5 client that runs in a web browser as shows well on tablets. They also have a native client for the iPhone. No native client for Android however.
Do.com adds to many choices for the collaborative task management. I definitely wouldn't say it's the best one, but it will certainly work for the 3rd grade politics :)