Well I've tested this out, and there is a limit, and it ***** - dino balls.
Only 9 pages will fill up, as mentioned. When you add more via iTunes, it just doesn't show up on Springboard, so you can't use them.
Also, it's all downhill from there. If you happen to reboot your phone with more than 9 pages of apps OR webclips, you will be stuck at the apple logo - forever!
A restore & re-install (with less apps) will fix it up, but that's a pain.
If you've jailbroken your phone, and have ssh installed (and running), you can get to the apps directory and drag the extra apps out, and use terminal to do a clean reboot.
Or if you're patient enough iTunes 'might' see the 'stuck at apple logo' phone in-between the Springboard kernel panics & restarts, and you can de-select some apps & re-sync.
All & all pretty shoddy. I don't care about the 'why so many apps' question - it's irrelevant. What if it was just 2 pages of apps & the rest webclips? This is definitely a problem that Apple should look into & fix.
What if your Mac had a 500gig hard drive, but as soon as you install more that 80 apps it won't boot up anymore? Crazy.
Common Apple, you give us the world sexiest, smallest handheld computer, running an os based on 10.4.8 (oh and it can make calls too!), and you expect us not to use it to its full potential?
So in short, if you have more than 9 pages of apps and/or webclips, don't reboot your phone. It will be stuck at the apple logo (springboard is not loading).