From what I'm hearing, Google isn't getting talent at below market prices. What's not immediately clear in the acquisition price is the ridiculous retention bonuses Google's paying to keep these people on board. In fact, I think they're paying through the roof for talent already... no need to for competitors to bid up the price.

Competitors can bid, not with the intent of winning, but with the intent of driving up Google's price. This certainly happened to Cisco. The retention bonuses are a way of making a side payment to the entrepreneurs that bypasses the investors. Competitors could also offer retention bonuses.

How can you bid without risk that you will have to pay?

You can't, but there is a value in the underlying team and product: winning and overpaying from time to time still resets the market when a competitor is making many acquisitions. It also gives you more access to the deal flow, because now you are seen as an active bidder, which in turn offers market intelligence from being able to interact with all of these early technology firms.