I live in central London, UK (you can assume signal strength is pretty high here). Obviously I've also used my phone plenty of other places. In daily use I've not noticed any call or data issues with either the 3G or 4. Not that I've been looking for any, especially
Having just checked, seems I can drop my bars down from 4 to 3 by putting my thumb across the famous gap.
Where do you live?
Have you had any other network issues -- dropped calls or an inability to dial out -- either with iPhone4 or a previous iPhone model?
Is there no effect on your call or data quality (either as reported by 'bars' or subjectively experienced) when touching 'the gap'?