Have you ever experienced heat issues with your Core Duo? I heard that they had a problem with that and that the Core 2 Duos did much better at staying cool.
Well, honestly, it does get warm under the laptop....hot...no. If the laptop is on my desk, then them bottom is just slightly warm. From the temp. monitor on my widgets, I see that the enclosure is at 31 degrees celcius, which is quite good. If the macbook is on my lap, or on my bed, then yes it is a bit warmer, but not to the point that I can't handle it.
Now, it does get warmer as you do more processing, such as photoshop. I notice that the enclosure warms up to 41-45 degrees celcius, but again, it's not to the point that I can't handle it.
A co-worker of mine has the C2D version, and yes his runs a bit cooler, but for me, the $500 savings was much more worth it. Most of the time my book is on my desk or table, so the heat does not play an issue.
Hope that helps.