"We can't do them justice," is Relic's official statement, I believe, but if they can do Necrons, they could do 'Nids. They wouldn't be THAT difficult to reproduce in a game. They'd have a central, upgradeable mega-hub, much like the Necron Monolith (except it'd be a Norn Queen, on loan from a ship). For resources, you could elect to have a mobile synapse unit become immobile over a strategic location, thus simulating the Hive Mind effect while giving Tyranids a reason to capture locations. Also, it'd be a nice effect if creatures like Ripper swarms gained a points value as they fought and killed, only to be returned to the Norn Queen to be harversted for resources. It's not a perfect fix, but it'd work well for an RTS. I do hope that Kairon's right about the Zerg not preempting the Tyranids' role in video games, because they're really an interesting piece of fiction with a lot of history behind them.
But anyway. Starcraft!