I've just visited a local music store with a decent display of ukulele's and measured them with the consent of the shop staff. As these were all brand new instruments I wasn't expecting to see much of a belly or dip in any of them and overall they were pretty flat. What I'm going to describe bellow is all pretty minor or slight.
Concert size: over sound hole/ bellow bridge.
1,.....................flat.................flat
2......................flat.................slight rise
3......................flat.................slight rise
4......................flat...................flat
5.......................flat..................flat
tenor size
6.............slight dip..................slight dip
7................flat.......................slight dip
8..............flat.......................slight rise
9..............flat.......................slight rise
10,,,,,,,,,,,slight dip...................flat
11...........slight dip..................slight dip (times 3)
14............flat,,,,,,,,,,,,,,,,,,,,,,,,slight dip
15.........slight dip...................slight rise
16........slight dip....................flat
17.........slight dip...................slight dip
18.........flat.............................flat
19.........flat...........................slight rise
Baritone
20..........flat.........................slight rise
Bass Ukulele
21............flat.......................slight rise.
So there is a quick survey of brand new ukulele's in a shop ranging in price from $100 to $600
Only a few that you would really call flat. I wasn't expecting to see a lot of rise in them although I was a little surprised to see how many of them had the bridge just a tad lower than the sides. Any of them and probably all of them will move over time.
Anthony