An interesting blog post on the subject here:

NB. The numbers are pretty accurate in my opinion.

Update: Also of interest are some of the comments in this stackoverflow question. Some frank estimates of time by some of the developers for well known apps such as twitterific.

If  so, then you can reduce your monthly fee by £15 by simply converting to the Simplicity 20 tariff on Pay As You Go.

At £20 per month for 600 mins + 1200 texts and with unlimited internet, free cloud/open zone wifi and visual voicemail – this is actually a better package than I had on my £35/month (18 month) iPhone contract.  Crucially, this is a one month rolling contract so you are not tied to 02 for another 12/18/24 months.

Also, don’t forget to unlock your iPhone from o2!