The messaging application WhatsApp Messenger, made popular on the iPhone, is now available as a beta release for Android!

Direct & official download:
http://www.whatsapp.com/android/ or
http://beta.whatsapp.com/WhatsApp.apk

If you get an error while logging in into the app with your SMS code, saying you have to be a registered beta tester, just retry later that day.
Many user on different forums say that it seems to work better in the evening. (US time)
You have to do the verification only once, then the app is unlocked for you.
Have Fun!

(taken from xda forum; thanks to cgorki)

You may refer to xda thread for more info
http://forum.xda-developers.com/showthread.php?t=764163
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INTRODUCTION
Welcome to the WhatsApp Messenger for Android beta program! For questions and support, please send email to:

android-support@whatsapp.com.

When sending us bugs, please use this route inside the application:

Chats > Menu > Settings > About > Contact Us

This will guarantee that we receive application logs along with your bug report.

NOTES
The software will upgrade automatically. Please upgrade frequently to get bug fixes and improvements.
WhatsApp is not compatible with most task killer applications. They potentially drain your battery and causes message delays.
The SEND key is the enter / return key on the lower right of the soft keyboard.
The LED on HTC Desire only shines green
On Samsung Galaxy / Captivate these features do not work or are problematic: contacts menu item, send audio message, contact us

NOT IMPLEMENTED
Emoticons / emoji
Facebook address book items are not available to WhatsApp
Broadcast message
Send / Receive contact.
Chat wallpaper

LATEST CHANGELOG

1.0.7652
fix two bugs related to message deletion and message ordering.
1.0.7619 was a bad build. Our apologies.

1.0.7619
do not leave audio files at the root of the sdcard when using the built in recorder
fix conversation list display after message deletion
add enter key interface setting
fix crash related to broadcast message
fix crash related to loading messages
fix five miscellaneous crashing bugs

1.0.7548
fix registration problem resulting in un-chattable contacts
video messages now have a thumbnail for easy recall
fix crash bug while forwarding a message
fix crash bug while broadcasting a message
feature: Add WhatsApp Contact Shortcut from home screen
improve offline message notifications -- one beep for offline messages
fix one crashing bug at startup
fix three miscellaneous crashing bugs

Changelog Link - http://beta.whatsapp.com/test/7/WhatsApp.html

This post has been edited by hnr2802: Oct 7 2010, 04:13 PM

before this the apk are not shared publicly. furthermore, you have to email them to let them activate the app for you.
in your case you only have about 60day+- to use the app after that if you still wanna use it u gonna have to pay USD$1.99/year. it hv trial period of 90 days
btw, so your Contacts menu working? what does it actually do?

This post has been edited by hnr2802: Aug 29 2010, 12:59 PM

sorry because trying to explain it to u. oh well. i guess i dont need to pay anything my license from whatsapp blackberry still intack. the app stated Paid

This post has been edited by hnr2802: Aug 29 2010, 01:24 PM

when chatting it reduced my battery life but not alot. but when idle no significant changes.

but a little bit lag when replying chat on my galaxy s.