Granted, Gandalf does not yet know what the ring truly is, and even after the incredible struggle to give up the ring, clearly Bilbo still does not recognize the magnitude of its danger. But Frodo was not given a choice in the matter, and neither Gandalf or Bilbo appear to feel any guilt about inflicting him with it.
Gandalf never actually touches the ring does he? In the book, I mean (I'm sorry, I haven't done my homework yet--too busy with work).
I wonder, too, if he figured that the ring would have no real effect as long as Frodo never put it on or kept it with him. If he just had it in the house, hidden away somewhere, then it wouldn't be a problem--at least, maybe that was Gandalf's rationale.
The way Frodo responded to the ring--putting it away and never using it or taking it out--seemed to justify Gandalf's trust that the ring would not tempt him and would not have an effect on him...until and unless he used it.
-penny
But it's sort of a least-worst situation, isn't it? If he takes it, the world is a smoking ruin. If he picks it up with tongs, withstands the temptation to touch it, tosses it in the sea, it will turn up again.
But looking at Bilbo, who's had it for decades, it had become addictive, but had not utterly corrupted him. And he was able (if barely) to part with it, in the end. After DECADES of ownership and use.
And hehad no intent of Frodo having it that long.
So I suppose, having said all that, letting Frodo keep it was the "least-worst" of many bad scenarios.
Dan