This is a little misleading. 64-bit applications run as 64-bit regardless of whether your kernel is 64-bit or not. For example, AfterEffects CS5, which is 64-bit, can see and access all 6 GB of RAM I have on my MacBook, even though my kernel is still 32-bit. Go on any MacBook running Snow Leopard, fire up Activity Monitor, and you'll see several apps running as 64-bit. The rest of the OS is always 64-bit (except on Core Duo/Solo machines) under Snow Leopard.
The main benefit of booting a 64-bit kernel is to allow it to address more memory and have more RAM itself for managing the overall RAM. However, unless you have more than 8 GB RAM, this isn't necessary. So while that is a valid difference, it doesn't mean much to the end user.
That's what I thought, I was pretty sure it couldn't address more than 6GB of RAM, I didn't think about it being because of a 32-bit kernel, though.
It's worth mentioning that Windows does not offeer that luxury. Windows XP/Vista/7 32-Bit will only execute 32-Bit applications, BUT I'm pretty sure you can install Windows 64-Bit on a Macbook. It has all of the hardware capabilities, and as I understand it, it's an issue of not having 64-bit drivers for a lot of the hardware, plus Apple locking it out on those models (the same reason my first 64-bit CPU powered PC didn't run a 64-Bit OS at first, no drivers!)
If you have 8GB of RAM, I'd go ahead and get the pro, so you can use all of it, however, 8GB (depending on what your doing) might be more than you need.
I have 4GB on my PC, which I spent $1,400 building, 4GHz Quad-Core CPU, HD5870 (at the time, fastest single GPU graphics card), DDR3-21800 RAM, 790FX chipset, but only 4GB of RAM. I don't need more! I do some gaming, admittedly not much, but I play games like Battlefield Bad Company 2, which push a whopping 2GB including the OS. At idle, my PC uses about 1GB of RAM (and that's Windows, OSX will be much less). I have a dual-monitor setup that I use for photoshop, 21MP DSLR photos, even several at a time for panoramas, also rarely gets close to 4GB. (I keep a little desktop gadget on to look at usage). Personally, I think massive amounts of RAM is overrated, currently, and probably for the next year, 6GB is enough for almost any task. However, if you think you'll use it, like dozens of high-resolution images at a time (you need a MBP if your doing that anyway, it has a higher color gamut), or maybe compiling tons and tons of code (then you need a faster CPU, like on the 15" MBP or something). Personally, anything a user who is considering a MacBook, and isn't dead-set on a Macbook Pro, shouldn't need more than 6GB of RAM.
I'm not saying it's not capable, and it's just as fast as the MBP, but there are very few uses for that much RAM currently, and personally, I wouldn't go near any notebook with those sort of uses if I could avoid it.
I'm getting the MacBook here soon, I'm getting some money back on my taxes, so I'm gonna use that, grab a 128GB SSD, 4GB of RAM, and a USB enclosure for the 250GB HDD. I'm choosing it over the pro for Price, aesthetics, temps (I did find out, the MBP runs about 20F warmer in most spots over the MacBook, most surfaces on the MacBook stay under 100F, except for the hinge at 120F, not too shabby).
Let us know what you do! BTW, you can stick your 8GB you already have in a MacBook, it just won't address more than 6GB (I'm not familiar with how OSX handles 64-Bit software though, does it address 8GB despite the fact that the Kernel sees 6?, if that's the case it's completely a moot point, NO 32-Bit program, regardless if it's on a 64-Bit OS, can utilize more than 6GB of RAM)
-John