Yes, this is a true statement. This has nothing to do with Verizon though. Rather, it's a CDMA issue. Voice calls on the CDMA network use the CDMA2000 circuit switched network (similar to GSM/EDGE 2G phone calling on GSM networks). EvDO, CDMA's version of 3G, does not handle voice communications (in fact, EvDO used to stand for Evolution, Data-Only until marketing got ahold of it. Now, it stands for Evolution, Data Optimized). As with GSM, when you are making calls on the 2G network, you cannot have simultaneous data. With GSM based 3G (UMTS/HSDPA/HSPA), both voice calls and data are routed on the same packet switched network. So, if you are using an AT&T 3G phone and start a call on the 3G network, you can talk and surf at the same time.
For many people, this is a severe limitation. For me, being both a Verizon and AT&T customer living in the Dallas/Fort Worth area (one of the most heavily congested areas for both networks), I actually prefer the way CDMA does it versus GSM. Since AT&T's network never seems to handoff 3G calls to 2G when I hit 3G dead zones, I deal with a TON of dropped calls, more so on my iPhone than with my BlackBerry Bold. With my Verizon Tour, I have far fewer drops (I have to loose signal completely to have a drop versus just loosing 3G). Even further, me working in IT, I know how critical QoS is. Part of the reason why there are so many dropped calls on AT&T is due to the fact that on the 3G network, calls are competing with data for bandwidth. With so many full-internet phones on their network (such as the iPhone and others that can fully access the internet without going through proxies), running out of bandwidth within your tower is a real possibility. When this happens, you drop as well. For me, having a separate network for voice and data is critical, even if it means that I can't surf while talking (unless I'm on a Wi-Fi network, which the majority of the time, I am).