You're right, of course... the background is realtime-rendered
What is it that you're trying to do, out of curiosity..? Your initial post mentions volumetric 3d engines on PPC/Palm/Java platforms. Imho, if you're prepared to nail your game down to more specific requirements, you're more likely to get better results. Both in terms of supported platforms (e.g 3d performance on Java would be pretty weak, tho might interest you..) and the game type itself (e.g for a FPS-> indoor/outdoor levels, sprites/3d enemies, lit/unlit/dynamic lights, etc).
For the engine itself, as has already been said, your choices aren't great! From what I've heard though, what may lack in raw rendering speed, it makes up for in terms of development tools, support, etc which can really speed up the dev cycle, giving more time to spend on the game itself. In terms of performance though, the irony is that the coolest 3d engines are being done by independent game developers atm... who don't really have the additional resources required to handle engine licensing (documentation, support, fixes, demos/tuts, etc). And, for an engine as complex as the 4Sight engine (used in Gang Warfare), that adds up to a lot of extra work!
Diesel: I'm not really one to publicly bash companies... but anyone seriously considering paying the fee asked for dieselengine should email me. For half the money, i'll write you a way faster renderer. In HTML.
(Then again, they probably offer documentation and support for their engine, so its a tough call I suppose...
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