As @PhilipKendall said full-time autofocus isn't possible with DSLRs. This is due to the fact that when shooting video the mirror is raised to allow the sensor to record footage. The autofocus sensor relies on the mirror to reflect the image from the lens on to it. Basically if the mirror is up the autofocus sensor isn't getting information.
There are however cameras from Sony labelled DSLTs. The difference between the R and T is that the R stands for reflex whereas the T stands for translucent. The mirror in an SLT is in a fix position all the time. This is possible because the mirror is translucent. In Sony cameras I believe that ~33% of the light coming is reflected on to the autofocus sensor while ~66% is always hitting the sensor. This means that not as much light hits the sensor but you get full time autofocus which allows you to record video while leaving the autofocus on or shooting up to 12fps(on certain models) with each frame being separately auto-focused.
If you really want full-time autofocus you would need to switch systems which could be expensive. I am a Sony shooter(A77) and can honestly say that the autofocus is fast and pretty accurate.