As Jared said, they loose their toxicidy in captivity. However, along with all amphibians, they are not great handling pets.
They do not tame, are not very affectionate, and, the most important reason if you want your frog to live is that frogs absorb everything through a pad on their bellies. Your hand produces sweat along with other chemicals that are not good for the froggies. Plus, if you were gardening, and had spread cedar chips, and fertilized the plants, you would get instant dead frog. I own two Whites Tree Frogs, and though the "Goldfish Rule" also applies to them, I have to say it is hard not to hold them for a bit- there so cute, and plus, I give them weekly health checks, to check for any problems. It is hard to do with a WTF sience they are nocturnal, and the only time to see them is at night when it is too dark to see them clearly. You should be able to see them bounding around through the glass, though.
I know, trying to keep hands-off is quite difficult, but they are still worth it- or at least my WTF's are. (I dont own any dart frogs, but an looking into them)