The i7 620M in the MBP and i7 860 in the iMac share the "i7" name because they occupy a similar place in their respective lines (specifically, the upper end). They are actually rather different internally, although based on the same basic design. You can look up "Lynnfield" and "Arrandale" on Wikipedia for the nitty-gritty, but for starters, your iMac has a quad core, the MBP has a dual core.
Also, the GT 330M is considerably slower than the 4850. I don't know what kind of stuff you run (you didn't say) but the performance hit would be significant. Probably 70-100% slower in heavily-threaded, CPU-intensive stuff. The drop in gaming performance will probably be bigger. Not to mention the howling of tiny fans and the fear that your machine might suddenly blow up or melt, since it's a laptop and no one ever feels safe doing heavy stuff for hours or days on end with laptops. 😛 I'd say the same thing about the iMac, but the thing has gotten so damn big it actually has a desktop-sized heatsink in there now. Lastly, there is the lost screen real-estate, going from 2560x1440 to either 1920x1200.
If you want portability, think about a MacBook, iPad, or netbook, and hang on to your iMac. The MacBook is the most expensive, but it has the advantages of doing everything your iMac does, only slower, plus it can use your iMac as a second display if you don't feel like moving files over to work on something you have on the MB.
