Camera glass scratching
You'll have seen Nokia's official statement on this issue yesterday. Essentially, the problem is that the protective pad is mounted too close to the plastic camera lens cover, causing dust to get ground in and produce scratches. There are unofficial fixes on the 'net which involve just replacing the sliding mechanism with a China-sourced replacement, but the official solution, shown below, involves completely replacing the N97's plastic rear assembly, of which the camera protection slide is just part:
The new slide is mounted a millimetre or so above the camera 'glass' and shouldn't cause the same issue.
How to tell if your N97 has the old design: You'll know - you'll have scratches already. If your lens is clear of physical scratches then relax, you're OK. (If you look very closely then you can tell the two designs apart - one has a visible gap between slide and camera glass.)
Poor GPS reception
Rather less visible an issue, poor GPS reception is the biggest showstopper for owners, with Ovi Maps becoming just about unusable. The problem is not with the antenna itself (shown below for interest sake), but with the way electronic interference from the N97's PCB degrades the GPS signals being received.
(note the original, scratched, camera glass is also shown clearly)
The official fix is to install a new antenna, with an additional shield near the point where it attaches to the main PCB. Correctly installed at the service centre, GPS signals are then improved by "2 to 4dB" and this makes all the difference in the world - I was able to navigate a 30 mile journey with no a single loss of GPS lock and with faultless directions.
How to tell if your N97 has the old, unshielded design: You'll know - Nokia Maps/Ovi Maps will be useless in navigation mode - the lock will be lost very frequently. With the latest Ovi Maps 3.3 beta, you'll be spared this message but your reported position will be badly delayed. If Nokia Maps/Ovi Maps are working fine then relax, you've already got the new design.
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It's not known when Nokia changed the design of either component in the N97 production run - my guess (and that's all it is), for each of the two design changes above, is that it happened after a month or so. Which means that if your N97 was made since about mid-July 2009 then you'll be OK. But, as I say, if either of the two issues actually seem to be happening for your phone then just make an appointment with your local Nokia 'Care Point' (here's a directory for you to check out, worldwide) and they'll sort it out for you.
Steve Litchfield, AAS, 9 October 2009