I just swapped out the 512MB dimm in my MBP for a matching 1GB. The system profiler shows 2GB. Is there a way to see if I'm getting the full bandwidth for the dual channel pair?

Well, the CPU can't use it for a start, so it's probably only of limited relevance

really? elaborate please.

There was talk on the IRC channel a week ago or so. Apparently the extra bandwidth goes to anything but the processor. The GMA950 in the MacBook can use the extra bandwith for graphics. I wouldn't be surprised if it would allow other devices that use DMA to access the memory though.

The Core Duo is on a 667MHz FSB, at the same width as the RAM, I believe. That means that the maximum data rate the CPU can utilize is that of a single channel. Of course, DMAs from the video card or other devices could utilize additional bandwidth, but the chance that that occurs at the same time as the CPU is trying to use 100% of the bandwidth is small...

When the machine has a GMA 950 and is using system memory for graphics, there is a substantial benefit to the dual-channel, since the graphics imposes significant additional overhead.

That is why Apple is shipping a single SO-DIMM and an empty slot in Core Duo machines with dedicated video memory, and matched pairs of SO-DIMMs in the machines with integrated graphics.