Well my deck is configures so that a rolling cart would be useless...2 steps here, 2 steps there....wouldn't work.
But have a friend who used a tea cart on her patio...it was used as a bar, and could roll where the party was and move clost to the door to unload.
Another friend used a tea cart both as a bar and as a coffee and dessert server. The cart went from the kitchen where it was loaded with glasses bottles booze, mixers stirrers etc and rolled out to the screened porch. And in the winter it made a dessert and coffee server. It had big wheels to roll easily and drop leaves that, when flipped up made a decent size table to a bar...or dessert.
AND...when they didn't have company....they would set the cart for dinner for the 2 of them and wheel it from the kitchen to the TV room and use it as a portable dining table.

I've seen some interesting deco-like dim sum carts:

http://www.gsw-usa.com/img/p/15-62-large.jpg

My back yard doesn't permit any carts, because there are too many steps.

If I did use a cart, it would be general purpose - not just for tea. I'd want to be able to place teapot and cups in a protected place, like a closed cabinet or a shallow well, so that if the cart banged against something the crockery wouldn't fly off and shatter. I'd want some provision for warming food, even a sheltered compartment that accepts a Sterno tin. There should be a drainable bin for ice, essential for a cocktail cart. Note that the well, warming tray, and ice bin could all be the same thing. Bottle bins so that tall things (wine bottles, Campari bottles) can be carried without tipping. A little drawer for corkscrew, opener, and other things that should stay with the cart all the time. The wheels would have to be soundless, like large diameter soft rubber or urethane, and not get locked up by a pebble. I hate hearing a cart rattle across a patio, metal and dishes and glasses clattering and clanking on each other. Child safety requires that the cart be wide and heavy enough to be pretty stable, without exposed shelf tiers that look like ladders to little Tarzan. Stainless steel or other rust-proof material. Wheels right at the corners for stability, locks for the wheels or better yet automatic locks that are release when you press the handle (think airport baggage cart). Spats over the wheels would be cool if feasible.

I like Linda's idea of a multi-tasking tea cart, maybe with folding leaves so it could be used as a small eating table for two? How about a 3" to 4" deep tray that sits on top of the center area of the cart to contain glasses and pitchers, then they would stay put when wheeling the cart around? This tray would lift off when using the cart for dining. And maybe a curved bar handle on each end so you could push from either end and perhaps fold a small table cloth over the handle or hang grill tools from the handle?

Underneath the top of the cart, perhaps stemware slots to hold a few wine glasses? And a narrow lower shelf between the wheels to hold the ice bucket, etc.?

Thanks for all of the ideas - they are all very helpful, and I will try to keep them all in mind as I draw the cart. I will probably make several variations, although I do have a general design idea for the basic parts, but the functionality is what I was questioning or unsure about.

The first one will be going to a Russian family's summer country home in Provence, and my boss is doing all of the interior as well as the extensive exterior furnishings. Their terrace will have a hard, flat surface, and so it will be easy to wheel the cart about, but I'm still not sure about the wheels. I think the house is new construction.

I like the tea carts with folding leaves, but we will not be doing this with this cart, partly because the outdoor dining table has removable leaves, and there will be several outdoor tables. Today I was finishing up on the dining chairs, but they may have some tweaking to go yet. The finish on the tea cart will be polished or brushed stainless steel with an outdoor wood, exact type to be confirmed, but that will not affect my design direction.

Lars

Removable tray(s). One on top that nestles in, and one that would slide into a lower level.