Assuming this gen will last until the end of 2012. For Steam, a Costumer is an account with at least 1 bought game.

I know it's hard to believe Steam can surpass a console, but the truth is that Steam keeps getting costumers faster and faster, and right now it has over 25 millions costumers!

In May 2007, Steam had 13 millions costumers.
In October 2008, Steam had 16 millions. Increase of 176k/month.
In January 2010, Steam had 25 millions. Increase of 600k/month.

Too many reasons why Steam is becoming so popular. But, one reason is worth a mention: Steamworks. Steamworks is a FREE piece of code that devs can put into their games, adding many awesome features. It is also used as DRM for the Retail Versions of games, where you'll have to log in just once to your Steam account so you can play it, and forcing game owners to register on Steam and become part of the Steam family!

Steamworks is so popular with devs that many high-profile games like Modern Warfare 2, Empire: Total War and Dawn of War 2, and also Valve's own games, among others, are choosing Steamworks as their DRM.

In 2010, Steam is reaching to new heights, with amazing games using Steamworks like CIVILIZATION V, and the release of Steam on MAC and LINUX, plus Epic integrating Steamworks on the UE3 Engine! Here's what 2010 brings to Steam Costumers:

• Portal entirely free until May 24th! All free copies are real copies, so you are a paying costumer even with only Portal!
• Steam release on MAC! A huge move that millions of MAC users have been waiting for.
• Steam will be on Linux! It is Official, with Valve confirming it to several websites.
• Epic puts Steamworks on UE3! Now, games made on the most popular Engine can choose to join Steam so easy!
• Big games releasing with Steamworks:

1. Civilization V
2. Napoleon: Total War
3. Dawn of War 2 - Chaos Rising
4. Football Manager 2011
5. Supreme Commander 2
6. Metro 2033
7. Alien vs Predator
8. Just Cause 2

Plus, continued sales from older titles (because PC games never stop selling) with Steamworks like Modern Warfare 2, Left 4 Dead 2, Football Manager 2010, FEAR 2, Dawn of War 2, Empire: Total War, etc.... will be sure give a few more millions of costumers to Steam.

What happens if Steam starts getting 1 million new costumers ever month?

I have only 2 questions

1) If a person is forced to get a steam account in order to play a retail game, should that really count?

2) Have we got any idea as to the attach rate of steam?

1- Yes, because the game you bought on retail will be on your account.

2- no idea.