A lot depends on space availability, and on the quantities of sheets/panels you need to handle Greg. I'm no expert, but i looked at this problem recently, and concluded that i couldn't afford the floor space taken up by a cart - but bear in mind i don't use a lot of plywood, only the odd few sheets now and then.
One issue with 'leaned over' storage is that it can very quickly get next to impossible to get out sheets buried in the sandwich. Sheets may warp if not well supported by a flat surface too.
I've gone for a wall mounted 'stood on end' (8ft high x 4ft wide) rack to minimise the footprint, have bought a 'Gorilla Gripper'
http://www.gorillagripper.com/ (which works very well) for single sheet handling, and will build a small single panel 'roller skate' like this if needed:
http://www.rockler.com/product.cfm?page=5188
One rack idea i've gone for that may work for your cart (instead of leaning the sheets against both sides of a central pillar, which makes removal of inner sheets very difficult, and makes it easy for the sheets to fall outwards) is to place sloped support pillars at both sides of the rack floor - so that the sheets while leaning outwards against the pillars sit in the central channel they form with the floor.
The sheets can't fall out, but can easily be removed from the ends. The big benefit (provided you don't fill the rack past the mid point of the floor) is that you can 'walk' sheets from one side to the other - to get access to buried sheets.
To save space, and to make it easy to move sheets I went for a very shallow 4 deg slope on the pillars since my rack is fixed, and made the central floor from slippery PET plastic bent upwards along the centre line in a 'V' making a 90 deg angle with the sides. You may need more of a slope on the sides for a cart though.
I made the pillars/sides in ladder form so that sheets lying against them can't sag and warp.
The plastic floor is gilding the lily a bit (i've a friend in the plastics fab business), but the floor arrangement is worth thinking about to ease handling, and reduce the risk of splitting veneers away when sliding ply sheets in and out on edge.
A decent radius at each end of the floor is at least advisable, as (in the case of a rack) is raising it a bit above the floor to avoid problems with e.g. water leaks/spills...
ian