[Lizard]

I don't know of a way to do it exactly like that (I'm not the Outliner guru JamesM is!), but I've found a semi-hackish way to get similar behavior. Depending on what you're doing, it might or might not be worthwhile.

Add a new column to your document, of type "Number" and summary "Total".
Add a 1 in your new column in each row that doesn't have children (each one that isn't shaded gray). OmniOutliner will then do the sums for you.
It's easy to add the 1 for all the items you have in your list now.
Here's the hard part (unless you're using a script to add items, like I do): you must also remember to do this when you add new items later. This might prove too hard, and leave your sums out-of-date.
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Another possible work-around that doesn't require you to maintain it: Turn on legal numbering for all rows in the document. Then to see how many items are in Groceries, simply look at the number of the last item in that section. They'll be numbered like so:
5. Groceries
5.1 Apples
5.2 Milk
5.3 Bread

So you look at Bread and see "Oh, I have 3 grocery items to buy."