Here is the vSphere 4 Reference Card.
The card is available under a Creative Commons license, so feel free to copy and distribute this in paper or electronic format. Just check back frequently to www.vReference.com for updates, or subscribe to the RSS feed to be the first to know.
If you spot any additions or updates you think are needed, then let me know and we can improve this resource for everyone.
(If you want the latest card to support vSphere 4.0 (v2.2) you can grab it here)