Telstra ones have an internal aerial modification
It's the network frequency of the Telstra Network (but not Voda etc) that was more compatible with the 3GS. not a modification. I think the new iPhone 4s have a wider spectrum of networks they can access at higher speeds. Try whirlpool.com.au for more info.
As for apps -
Sunrise Sunset Pro. very useful for determining Last light etc.
World view - webcams all over the place
Aeroweather - Latest TAF/TTF Metars from any airport also Last lgiht etc
Iports - same as above but with RWY info
Time Calc. A time calculator.
Touch PLan - a quick and dirty flight planner
Mental Case - combined with mac software, a fantastic test / notes / study tool. Brilliant and easy to make questions with pics and diagrams.
There are a bunch of pprune threads on this - I'm sure I've typed it a few times! Search.