As for the laptops vs desktops... I would wager a guess that its a big security concern. Laptops stay in secure areas, aren't moved, etc. There is a whole new layer of security involved in laptops as they leave the office and go to airports, homes, car trunks, etc. This is more than likely intentional.
You're more than right about the security ramifications, but this is 2008 -- we've solved those problems already. Fortune 50 companies give laptops to anybody who has any reasonable need for portable computing. Security risks are mitigated by using off-the-shelf, driver-level encryption. All VPN tunnels are encrypted using multi-factor authentication. Big companies are selling these solutions and other big companies are buying them. It's simply NBD anymore.
The real problem is just that the government is usually not very savvy at anything other than bureaucracy. Hell, most high-level officials can barely use a computer. This administration will begin to turn that around, however, as they won the election -- in part -- because of their technology savvy. The generation that grew up using the internet is just turning old enough to run for Congress, too.