Trev Not all guitars have a convex top. Martin is made flat without an arch. It is literally a "flat top" guitar. I found this out after thinking I had a problem with one. Brand new, they are flat and even concave in spots around the bridge. It is how they are made.
Actually I was looking at my guitar the other day and it looks pretty flat as well. I think I might have been talking rubbish about the slight curve. Sorry about that folks - I don't know where that came from. Maybe I was thinking about the back of the guitar, not the front.
Just as another note of interest, I think most classical guitars are also flat--but I am guessing now. The three I have are flat anyway.
I think most brand name and custom steel stringed guitars ARE arched. Martin is the only one that I am aware of that is flat. There may be others out there though.