According to iPhone Atlas, several users have reported problems about usage of 3G and GPS at the same time. This might be an issue espechially for applications like Maps that use GPS and 3G radio at the same time. Still, it seems a bit odd that GPS and 3G cancel each other out while EDGE and GPS will work anyway?
Let's have a look at the situation. First of all, Apple put the GPS antenna at the top edge of the phone and the GSM/UMTS antenna at the bottom, placing both as far away from each other as one could. This is a good idea and it seems that the fault isn't by layout design.
As EDGE is an enhancement to GSM, we should have a look at all the frequencies involved. That are GSM, UMTS and GPS frequencies.
Apples site states UMTS/HSDPA (850, 1900, 2100 MHz) and GSM/EDGE (850, 900, 1800, 1900 MHz). GPS frequencies are 1575,42 and 1227,60 MHz. So much for the obvious.
But wait! UMTS isn't just 2100 MHz! This is just a "label" for the frequence band used to run UTMS. As UTMS uses bi-directional communication, there is an uplink and a downlink involved, both running on different frequence bands. These are 1920 to 1980 MHz for the up- and 2110 to 2170 MHz for the downlink.
I played around a bit with that numbers above, but I coudn't find a overtone common to both UTMS 2,1 GHz band and GPS. Given this and the UMTS transmitting power of a maximum of 0.25 Watts (GSM has a maximum of 2 Watts!), it seems quite a bit unlikely that UMTS interferes with GSM...
The GPS chipset and the UMTS chipset and amplifiers are situated at opposite edges of the logic board (which also makes sense, considering the position of the antennas), so...?
Well, considering the following quote and the facts listed above, it clearly seems to be some sort of software related bug.
"Readers have reported, however, that turning 3G networking off, pegging a location, then turning 3G back on results in persistence of the accurate results."
Or, to put it the other way round, it seems that the issue isn't related to hardware in any way.
We'll see if firmware version 2.0.1 fixes this, which, according to rumors, might be around the second week of August.
[Update] Well, I just tested my iPhone using UMTS and GPS at the same time. I had an UMTS connection with almost full level and started Maps. Almost in an instant I had a GPS fix. So, whatever issue causes problems on other iPhones 3G, it is no deal on mine...
Lucky me, I'd say ;-)