Well for one thing, keep in mind that the website is not always up to date. For example, it is currently listing "Armoured Soldier" as an "in demand" job, when in fact the trade is actually closed until at least April 2010.
As for what it means, well, it certainly doesn't mean that these jobs are the only ones currently available. It simply means that at the time the website was updated, those jobs were the ones that were deemed to have the greatest need for recruits. I imagine this is determined via a combination of both training slots available, and the projected future ratio of trained personnel to positions required. I say projected future, because if the trade is currently under strength, but it has a whole whack-load of trainees in the process right now that may bring the numbers up to par, then there would be less need to recruit great numbers of that trade. The converse would also be true, as if there is a trade with a high level of anticipated retirements, and not enough people in the training system now to pick up the slack soon, that trade could very quickly become under strength.
In particular, the "in demand" label doesn't really say anything about the targeted number for recruitment in that year. It's more so about the ratio of how much they want to be getting in that trade versus how much they have been getting. For example, Infantry is not an in-demand trade, whereas Pharmacy Officer is. But in any given year, a great deal more people will enter the CF as Infantry than there are Pharmacy Officers in total! But because of the fact that we currently only have 37 Pharmacy Officers, and we should have 49, more focus is being given to try and entice those people out there who do have the skill set to become Pharmacy Officers. There is a shortage of people who want to join up as a Pharmacy Officer, there is not (currently) a shortage of those who want to join up as Infantry.
Also, I second FDO's suggestion of joining as a Tech. Might I suggest a Naval Electronics Tech (Radar)? Good times, etc.