Just a note of warning, users of the Twitterfon twitter iPhone client, changes are coming. The author posted a blog post about the upcoming Ad support for TwitterFon 1.5. The change (which just showed up as an update for me this morning) has quick add/remove, rich tweet view, and user lookup among the other changes. Looking at the Twitterfon page it also looks like there's now a "Pro" version of the app.
I'm not sure how I feel about this. I use Twitterfon because it's clean, free, and uncluttered with ads. On the other hand, the app author deserves a bit of a payday to reward him for the great work! Guess the simple answer would be to suck it up and get the pro version for (most likely, as it's not yet actually out it seems) less than the cost of your starbucks double no-whip skim latte :)
However, if you're a cheap bastard like me, maybe hold off on the upgrade to Twitterfon 1.5 until someone else sees how intrusive the ads are.