If you have to run OS X, I would say the new WhiteBook is by far your best option of those options you listed. The WhiteBook was just updated. It actually has a bigger HDD than the 13" MBP. It also has the 7-hour battery, nice display, Nvidia GPU, and now plastic unibody form factor.
I would make sure your MicroCenter option is the new "unibody" WhiteBook. If it's not, definitely look into paying $100 more and getting the new WhiteBook. The battery alone is worth it, but the design, display, and trackpad updates are great too. The new MB is by far the best value from any current Mac notebook right now. I actually think it looks better than the aluminum 13" MBP (I love the all white look instead of the ugly black keys and black bezel from Hell). If the MBA had aluminum capped keys, instead of these horrific black keys, it would be the most beautiful Mac ever!
I wouldn't buy an original MBA unless you could get it for around $400 or less. At $400, it could be worthy. A penny more than that just doesn't make up for all of its problems... in my opinion. I believe the value has to be compared against newer MBAs. If you can find a rev B for $800, that's a completely different story. I wouldn't buy a netbook over an original MBA. The original MBA has a full-sized keyboard, 13" display, and real OS X. I have a friend who just bought a rev B 1.6 GHz with 128 GB SSD for $650. In general, people pay too much for original MBA's because they don't realize how inferior they are. Since the newer MBA's are widely differentiated from the original MBAs, sometimes people sell 1.6 GHz rev B MBAs very cheaply not knowing their true value and compare them to the original MBAs. The MBA option definitely requires some patience to find the right rev B MBA for the lowest price.
Consider what $500 can buy you, an iPad. I think that will do most of what you want. Wouldn't multi-task, but you could do one thing at a time on it. Microsoft Office will be there as well as iWork. However, missing Flash would suck... but if you have a Mac mini, do you really need Flash? Yes, it would suck to not be able to browse to video sites and watch TV on Hulu. The iPad missing Flash is really going to ruin its potential, in my opinion. But paying for content or converting DVDs to iTunes would provide optimal viewing on an IPS display.
Also consider a PC ultraportable. There is another thread just made of the Dell V13. It's the size of the Adamo (weighs around 3.25 lb), and it sells brand new for $449 with Ubuntu or $549 with Windows 7, Core Solo CPU, 2 GB RAM, 250 GB HDD, and even 13" display (again beats out the netbook).
I would definitely select a new WhiteBook if I had to run OS X. A netbook is going to provide far inferior Hackintosh, display, typing, and performance capabilities. The iPad would be a decent choice if you didn't care about loss of Flash and lack of physical keyboard. A PC ultraportable could be a wise choice - Windows 7 is actually quite amazing, and I would actually consider a V13 if I wanted to save money or didn't want to worry about the kids destroying it or it being stolen or whatever. If you could somehow get a rev B MBA, in your price range, that is going to be your most rewarding experience for extreme portability yet MB-like function and performance.
Good luck whatever you chose.