According to a Google technician, Erica Joy, Google has submitted Google+ app to App Store and pending approval. Depending on Apple and some luck, we might see the Google+ app for iOS soon.
So, for those who are tired of using the iPhone Safari browser to go into Google+, do wait a while for the approval. I guess for those with Windows Phone 7, you might need to wait for Mango to access Google+ on web. And for the app on WP7, I guess it will be a long wait (prove me wrong).
If you do not have an invite to Google+, you might want to take a look at this.
If you do not what is Google+, you can look at this.