Yes, I currently am heavily invested in the Apple ecosystem with an iPod touch, iPhone, iMac, iPad, and a few other devices floating around.
Yes, I could be charitably called an Apple fan-boy at this point.
No, I won't stay with Apple if their build quality and/or experience quality goes down and someone has a viable alternative.
I don't have the money to throw around on lots of different devices, so I need to figure out what I'm going to buy that's going to last me 2 - 5 years (depending on the product and purpose). Right now, that's Apple.
When I'm ready to replace? We'll see then what the answer is. Five years ago, my answer wasn't Apple (it was leaning toward it, and the switch to Intel made it easier, but it wasn't there yet). Five years from now, my answer may or may not be Apple. It's up to Apple to convince me that they remain better than their competitors for me to stay.
Now, things are a lot harder. iPhone is still great, Android is doing really well, clearly catching up. Palm Webos is also good, and Windows Mobile 7 would probably be a worthy contender when it appears. BB is getting more consumer friendly, and Nokia have some great OS as well. Looking at Mobile OS, it seems you are right now, but in the past it was a lot easier.