This report clears a few things up and puts those fears to rest, also making mention of Nehalem's ability to run different memory configurations (including mismatched modules) without any hiccups.
The granularity is 64MB, in other words, the smallest size on one interleave would be 192MB, although it's unlikely that someone would end up with such a configuration, as the smallest DDR3 modules are 512MB. Again, this is good news for people planning to upgrade, as it's possible to run with unevenly configured memory, but according to Intel you get the best performance if you keep the same amount of memory in the same channel, although this doesn't mean the same size modules in each channel.
For example, if you own a 2x2GB DDR3 kit today, you can compliment it with a 2x1GB kit and split it into 2GB per channel for the best overall performance. This means that triple-channel memory kits might not be as popular as first expected and it could possibly upset a few memory manufacturers that hoped to sell a lot of triple-channel DDR3 memory kits.
Check out the full report here.