Yes, it's a limitation in CDMA that disallows the use of data while on a voice call. If you ever had dialup, you'd know how this works. If you haven't, I can understand. Basically, the call will go through, your data will pause (or stop, depending) and then you'll resume your data after the call is ended. From what I understand, you also still receive texts while in a call.
So, this means that when you're sharing your internet connection to your friends and your boss calls, they lose internet. Also, good luck on their "unlimited" internet.
Once again someone quoting what they don't know about. I have never been restricted in any way by going over what you call Verizons "unlimited" internet. Once for about 3 weeks I had no internet so I used PdaNet on my Droid X to connect my laptop. I was on there everyday with NO problems, no contact from Verizon telling me to stop, and no overages!!! Now when I had At&t that was a different story. I went over their 2gb plan often, and had to pay huge fees.
Also for those saying that the iPhone will suck on Verizon because Verizons cdma network is much slower than At&t's, well at least I can use Verizon's network without having to connect to wifi just to make a call. As a matter of fact I have wifi at home, but never use it on my phone because 3g speeds is fast enough for me.