Ice Castle from Frostburn

[phindar]

That's a more literal interpretation that I'd go for. Under Conjuration it says "It must arrive in an open location on a surface capable of supporting it," and under the Summon Monster entry it says, "Creatures cannot be summoned into an environment that cannot support them." To me, it's pretty clear that the intent of the rule is so that people don't drop whales on opponents. Otherwise you couldn't summon a whale at all, because the only enviroment that can support the whale is underwater, which isn't open, nor is it a surface capable of supporting it. (Of course, it would be a lot less ambiguous if under Conjuration is just said, "You can't drop whales on people.")

 

[Graf]

It would be in both the spirit and the rules to say that since flying creatures are "self supporting" (or can support themselves in air) you can summon them on anything (including nothing/air).
Likewise incorporeal creatures have a weight of zero, and can be supported by anything.