I specify sweet fruit here because I think there would different good answers for a savory or a custard based pie.
It actually depends on the quality of the shortening you're able to get. You might think they're all the same, but you'd be wrong.
When I was taught how to make pies, we used shortening, and the crusts were perfect. However, I was told that for home baking, the shortening you buy in supermarkets (Crisco, normally) just isn't going to cut it and to use Tenderflake (lard) instead. I actually verified this once and found out that he was right - using the exact same technique, the supermarket shortening just didn't turn out the way the "industrial" shortening did. The crust is always too mealy and dry and tastes "off" somehow. Unless a lot has changed in the past 5 years, lard is actually much closer to the good shortening that's being used in bakeries.
Butter would, obviously, impart a much richer flavour than shortening, but I wouldn't use just butter in a fruit pie. You won't get anywhere near the flakiness of shortening or lard. Half-and-half is a decent compromise, but the result is neither as flavourful nor as flaky as lard.
So, generally, I would stick to lard. You could take plor's suggestion and mix it with some butter, but I've found that the flavour and texture is very good with just lard; if you do decide to mix, be careful not to overdo it, otherwise you'll lose all the wonderful flakiness that the lard imparts (I'd recommend 75% lard).
Don't mix lard with shortening. That can only take away from all aspects of the quality, and unless you're worried about nutrition (in which case, why are you using lard at all, or eating pie for that matter?) then there's absolutely no reason to "taint" the lard this way.
P.S. Don't forget to add some sugar. A little goes a long way in pie crusts.
I've always tended to use butter in pastry, be it sweet or savoury. The trick in making it flaky is to minimise how much you work the dough, and trying to keep the fat from melting. A food processor is ideal for this, as you can pulse it to quickly combine the flour and fat.
You can also add icing sugar to a sweet pastry, which I've found can make a crispier pastry, and will hold a wet filling much better (when blind-baked)
I actually use about 2 parts lard (or shortening if you can't get lard) and one part butter. That way it is flaky but still gets some of the buttery flavor.
Lard makes flakier crusts, but I think that butter makes better crust because it imparts more flavor.
Like Rowland said: use butter, and don't push it around too much. If you don't have a food processor the old way to try to stop the butter melting was to dip your hands in cold water (then dry them) first...
I freeze the butter first and use a food processor - then freeze the mixture before rolling it out. Butter is just tastier than lard - I'll sacrifice a little flakiness for much more flavor.