Homes built on a crawl space needs ventilation! Right? Right! Notice I did not say "Needs vents", that's because a crawl space does not need the foundation vents (that go outdoors) if, and I quote:

IRC 2006, Section 408.3 Unvented crawl space

Ventilation openings in under-floor spaces specified in Sections R408.1 and R408.2 shall not be required where:

1. Exposed earth is covered with a continuous vapor retarder.

2. One of the following is provided:

2.1 Continuously operated mechanical exhaust ventilation at a rate equal to 1.0 cfm (0.47 L/s) for each 50 ft² (4.7 m²) of crawlspace floor area, including an air pathway to the common area, and perimeter walls insulated in accordance with Section N1102.2.8.

2.2 Conditioned air supply sized to deliver at a rate equal to 1.0 cfm (0.47 L/s) for each 50 ft² (4.7 m²) of under-floor area including a return air pathway to the common area, and perimeter walls insulated in accordance with Section N1102.2.8.

2.3 Plenum complying with Section M1601.4 if under-floor space is used as a plenum.
Source- http://resourcecenter.pnl.gov/cocoon/morf/ResourceCenter/article//131

So, the answer to the question "Can I really seal them up?" is, YES you can seal them up and you should seal them up once you have selected and are ready to install a continuous vapor retarder. The advantages of sealing the foundation vents goes well past the moisture control aspects. Every winter, that cold kitchen or bathroom floor reminds us that we need to get some insulation in the crawl space. Well, before you can warm anything you have to keep the cold away. Closing the foundation vents will make your heating bill drop, warm your floors, and keep the water pipes from freezing. DO NOT CLOSE YOUR VENTS IF YOU ARE NOT GOING TO SEAL OUT THE MOISTURE FROM THE EARTH. One thing to know about this subject, and any subject regarding crawl spaces, if you try to fix one problem without addressing the other "known" problems there is a great chance you will worsen your crawl space headache. Educate yourself first!

Matt