Here is the announcement from yesterday:
The Booster Packs can be given at anytime whether you are in a Steam game or not. It is currently unknown if there is a limit to the amount of booster packs you can get. But they are not very common; most users haven’t gotten more than one booster pack since the update went live.
Booster Pack drop rate is also affected by your Steam Level. For example if you are Level 10, your chance of receiving a booster pack will be 20% higher than someone who is under level 10. The chance is raised by 10% every 10 levels.
For more information on the Steam Trading Card beta please visit the official page, or the official group.
