Ok I know this is a dead thread, but just wanted to point out you actually do shield dots and can block them. This was one of the changes in 2.0 which is why in pve shield absorb was more beneficial because you can now shield all forms of damage.
Actually not quite, shield can now shield all kinetic and energy attacks. The vast majority of dots in pvp are internal or elemental dots. (there are exceptions, but the majority of the hard hitting ones are I/e)
So for practical purposes you can assume that you can't shield dots.