Is that the full standalone Safari or an embedded Safari within the twitter client? It looks like an embedded browser within the twitter client.
On my iPod touch, if I left my twitter client to go to Safari, I could not come back to exactly where I was in tweetdeck. But I have not tried 4.0 in this regard.
It's a fully functional browser that loads from the app, yes. All developers have this at their disposal. This isn't a problem precisely because apps rarely need to "leave" to display a browser. One just comes in modally, displays the content of a page, and that's that. If you were using a Twitter client that yanked you out of Twitter to show a site, you were using a bad client.
Then I want to email the link to that page, so I have to open the full Safari, then do the email link which opens my mail client, then close the mail client which goes to my home screen, go find my twitter app and open back up to pick up where I left off!
I like my iPhone but it has it's flaws just like any other device.
This is a free app. It's up to the app's designer and isn't endemic to the device or OS, unlike the inexplicable permanent presence of a physical back button on Android devices.
On my iPod touch, if I left my twitter client to go to Safari, I could not come back to exactly where I was in tweetdeck. But I have not tried 4.0 in this regard.