I think people here have missed the quiet (but reasonable) explanation for the "no source languages but ours": that, to do background processing, Apple's going to have to assume that it completely controls and understands the runtime behavior of an app at the Obj-C runtime level.

Other runtimes (even those compiled to Obj-C, because they require additional runtime machinery) would break their assumptions, and thus their chance to do the low-level magic required to really throttle down apps.

Disappointing, yes. Was looking forward to MacRuby, but that won't happen until GC reaches the iPhone OS.