CountSMS – iPhone SMS Counter (Updated: unnecessary)
Update (2011-06-02):
Of course it's been quite some time (I believe with iOS 4.0) since this has been rendered unnecessary. The official Messages app now lets you view the currently used characters. Good job, Apple!
If you ever attempted to send a text message on your iPhone, you will probably have noticed that it is kind of hard to estimate the amount of letters you have entered. Which is quite annoying though, seeing as a miscalculation might cost you money.
When I dug around the internet in order to find a capable app that could replace Apple's standard Messages app, I could only find solutions for jailbroken phones ("BiteSMS", I believe). My iPhone however isn't, and it won't be (at least not for a long time), so that wasn't an option.
A nice little application that doesn't integrate with your standard Messages app (sadly 
) but however does the job quite well (as in: count the characters AND have a landscape mode keyboard) is CountSMS (iTunes link). It is for free and you can use it with the clipboard of course without any problems whatsoever. It even has nice Copy and Paste buttons on the top corners! 
I know it's not an absolute amazing app, but it gets the job done (a lot better than Apple's standard Messages app! Shame on you, if I might say so )
Thanks for reading and I hope you can benefit from that.