I think if Atlanta's title hopes wane over the next few months, this is totally plausible. Allen could even return next year to Boston, potentially. They don't need his Bird rights as he won't be getting a salary increase. Josh Smith is in a lot of ways what Boston hopes Jeff Green could become, for about the price he would cost if he got there.
For Atlanta, if they made this trade they would be under the cap next year with the option of using their amnesty on Joe Johnson and completely starting over with Horford & Marvin Williams (1 year left).
Not saying it will, or should happen, but I think it's a pretty fair scenario..