Long version: The way I see it, there are 4 possibilities:
The first is just the null hypothesis: AS does nothing with respect to rend. Obviously your attack animation will receive a speed boost, but once rend hits a monster there is no difference.
The second is that AS increases the rate at which rend deals damage, but not the amount of damage it deals. This would correspond to a reduction in the base duration. For example, 10% total IAS would reduce rend's duration to 4.5 seconds but a base 50k weapon hit would still do 350k damage.
The third is that it increases the rate at which rend deals damage, but maintains the 5-second duration listed in the tooltip. This would correspond to an increase in the total amount of damage done by rend. For example, 10% IAS would result in a base 50k weapon hit dealing 385k damage over 5 seconds of rend, rather than 350k.
The fourth is a combination of the last two. Rend deals more damage and has a shorter duration. In this example, 10% IAS and a 50k base weapon hit would give you a rend that lasts for 4.5 seconds and deals 385k damage.
So, rend users... does anybody know what's up with rend and IAS?