You now have to upgrade to the second tier if you want to have more than 500GB hard drives or an SSD, so if you really want that you'll have to pay $300 more than the base model. Too bad the hard drives in these macs are so hard to replace yourself.
Compare the the ease of replacing your PC laptop drive or RAM - behind a snap-off cover.
RAM and HDDs in MacBook Pro machines are user-replaceable parts. No warranty is voided provided you install the proper stuff, and don't break your system in the process. I believe the manuals include detailed instructions for the replacements.
The RAM is _very_easily_ upgraded in an iMac. Apple's even designed a nifty door to make this process incredibly easy.
The only point of annoyance here is the HDD in an iMac. The design simply doesn't allow for the HDD to be accessed, and hence it's not user-installable. Personally, I'd never buy an iMac, and I very seldom recommend them.
You are correct about the hard drives on imacs and macbooks, but on the pro machines, those are pretty easy to replace as well.
It's not the same as the iMac (which is hard to upgrade iirc), but the MacBook (Pro) is a really nice machine to upgrade, and it doesn't void your warranty. Apple even has nice instructions on how to do it. (http://support.apple.com/kb/ht1270)
To upgrade the memory in an iMac there's 1 screw in the way.
To upgrade the memory or hard drive in a MacBook, there are 10 or so screws, but as long as you don't damage your device in the process it remains under warranty.
People like to make Apple out to be draconian about this stuff, but let's at least do basic fact-checking.
http://www.ifixit.com/Guide/Repair/iMac-Intel-27-Inch-Hard-D...
I pine for the old days of white G5's and original iMacs where they were a piece of cake to work on. That said, I don't pine for their bad capacitor problems...