Did you take a wrong turn because you were distracted by talking on the phone, or because Google Nav lost its data connection? In the former case, well, no help. But otherwise... if that's a problem (I have used it, but not much, having a dedicated in-car GPS with 5" screen, integrated into my car audio system) it's a general problem. It should be caching the whole route, and perhaps a zone around the route, to ensure that you don't get lost, even if there's no connection.
As for data and voice... yeah, Verizon uses the CDMA version of 3G, called EvDO. They change the name at some point, but this used to stand for "Evolution - Data Only". It's pure IP. So you might talk via a VoIP application if you had one, but there's no voice layer in 3G mode.
There are trade-offs. The big advantage of EvDO is that EvDO Rev A (which DROID and pretty much every cell in the network supports) does 3.1Mb/s downlinks (peak) over the same 2.5MHz channel (1.25MHz down, 1.25MHz up) as your voice link. So it was much the same radio gear and exactly the same spectrum as EDGE. So every cell does 3G.
In the GSM world, they're using HSPA or HSPA+ these days, which does have a 3G voice mode. They added that mainly to get around some nasty standard voice issues. In all CDMA and GSM 3G, there's a "soft handoff" between cells.. . you phone can remain attached to three cells at once, with the cells deciding where the call actually routes. So you never drop a working cell until a stronger one is available. On GSM voice, they do "hard handoff"... they drop one cell, then very quickly pick up the next. It leads to more dropped calls, but fixed in 3G.
The downside is hardware and spectrum. A basic HSPA connection gives you 3.6Mb/s down (peak), but requires 10MHz of bandwidth, 5MHz up, 5MHz down. So they needed not only all-new cell-tower gear, but new spectrum. HSPA+ peaks at 7.2Mb/s down, but works with two coupled cells on the came tower, for 20MHz bandwidth needed. This is why, by area, AT&T only has 3G in about 20% of their coverage area (it's much better by population, of course... you can get HSPA in most large cities, and by the summer, HSPA+ in about 40 cities in the USA).
HSPA+ can also do 2Mb/s uploads. But the iPhone only supports the original EDGE upload speed of 384kb/s... even the 3GS, which does HSPA+ downloads.