I believe one of the main barriers to people improving their skills is a feeling that they're either not prepared and thus not willing to risk the time and money on education or that they simply have difficulty in figuring out how to get from where they are today to where they want to be tomorrow.
A national online university would be a partial solution to both of these issues. It could focus on providing a decent quality, no frills education that people across the United States would be able to access to update their skills and to prepare to move on to more traditional colleges and universities. Also it would provide a way to provide everyone with access to college preparatory work without needing to worry about how reputable the local schools are. Of course, efforts would also have to be made to make broadband more accessible to the poor to make this work but that's a separate issue.
The online university would focus on two things, remedial courses and skills small businesses most need that can be taught online, principally business and computing courses. All courses would also be offered at very low cost to attract as many applicants as possible.
Remedial courses would be offered at no cost, provided that a student passes a course. If a student fails they would have the option of retaking the course or instead to try the next lower level, no charge will be levied if this course is successfully completed. This can be repeated until a student passes a course, with a number of well publicized exceptions for extenuating circumstances where a course could be dropped. These remedial courses would go up to the standard 100 and 101 level courses in subjects such as college writing or mathematics, which have come to grace the course lists for virtually ever university. The idea is to make going back to college easy, and to make it particularly easy and painless for those unsure if they are capable of doing the work so they can test themselves and gradually build the confidence they need.
The business focused portion should be fairly self-explanatory. It's simply a low cost way for business owners, or those wanting to become a business owner, to test the waters in a low cost way to see if college courses will benefit them. It can also provide individuals with a number of skills that will make them better employees.
To a certain degree this would compete with existing community colleges and online universities. Community college coverage is highly variable across communities however and existing online colleges aren't necessarily widely known about and can be expensive or of questionable quality, sometimes both. A national school would benefit from economies of scale as well as being easier to promote to those most in need of its services.
[As a side note, Edward L. Glaeser is promoting human capital as a driver of American renewal as well on the NY Times Economix blog. Being quite a prominent economist I thought I'd point it out. His vision is to sell this to the Republicans so it is likely to be vastly different from mine. I personally can't see how we can reach those most in need of a skills update without state intervention but it will be very interesting to see his take in his next post.]