A post on Planet Half-Life (thanks Frans) reproduces some quotes from Valve's Erik Johnson posted to a Steam forum thread discussing the role of Steam in the future of Half-Life. Here are the excerpts:

There will no longer be a WonID system, it will be replaced by the SteamID system.

When we convert the userbase to Steam, a single WonID will be valid for all of the products.

If you're purchased a product from us at retail, you're going to have access to all of our products via Steam.

No, you do not have to pay for Steam.

That is correct, you'll have to have Steam installed to play the most current version of Half-Life.