Must admit I cut to half-caff when I'm either a bit late or get a tummy upset in my LP, but the way I see it is that moderation is the key (and I'll be cutting to decaff if I ever do get pg). I mean, 500 years ago everyone in the UK drank low-alc beer, not water, as the water was icky and it didn't stop them having babies. Same with wine in southern Europe and tea in China. Although it was a lot weaker than we'd have now, I still think that if you "lapse" a little bit it won't have a huge negative effect.

(I once gave up all caffine for Lent - tea, coffee, cola, chocolate, the works (even the decaff stuff): that first cup of coffee at Easter was bliss! I will certainly be on decaff for any pregnancy!)