Texting SMS on iPhone 3G

Im thinking about getting a 3G iPhone to replace my Nokia 9300i. A quick test drive at the local O2 shop revealed that the SMS application doesnt make use of the 90 deg screen rotate, so you cant use it in landscape mode. I composed a test msg with two thumbs (which is fast & accurate on my 9300i) and it was slow, annoying and unreadable, due to the number of typos. The virtual key pad is tiny!
Is there an upgrade or aftermarket tool which allows SMS to be used in landscape mode with much bigger keys across the bottom of the screen?
Thanks in advance.
G4 Cube, Tower & Laptop Mac OS X (10.4.11)