UPDATE 28th July - The official app is available in most countries already. So download from Android market instead.
The official Google+ Android is only available in certain regions. https://market.android.com/details?id=com.google.android.apps.plus&hl=en
But, fear not, XDA member sacredsoul, has got his hand on the unofficial Google Plus APK for international Android users.
I've tried it on my Galaxy S i9000, and it works perfectly fine.
Get yours from XDA forum http://forum.xda-developers.com/showthread.php?t=1148305