Just install PdaNet from Cydia. Do a search on google for full instructions on how to use it.
Basically you just create an ad-hoc wifi network between your iphone and your mini and then run PdaNet. Bingo, instant tethering :-)
Before the last iPhone update I used the built in iPhone tethering. However, the last iPhone update, removed the official tethering option from O2 iPhones (unless you pay an extortionate amount to enable it - Â£15/month I think).
---------- Post added at 01:15 PM ---------- Previous post was at 12:59 PM ----------
Just searched google and it appears there are other methods available now for enabling tethering.