I RP frequently from my iPhone, and with the holiday season upon us, I figure there will be lots of folks with new iPhones and iPod Touches in the RP, so I'm going to write up a little guide about the things I've learned by RPing on an iPhone. I will refer to iPhones and iPod Touches together by lumping them in as iPhones - but know the only real difference is the iPod Touch users can only RP where there is wifi.
First, the iPhone is an amazingly productive device to RP on. You always have it with you, so you can stay in touch and fire off messages quickly and easily. There are some great Twitter applications available for the iPhone as well, and that greatly helps with RP.
Here's how I use the iPhone for RP. I'm a fan of the native Twitter app (simply called Twitter for iPhone), and it works VERY well and it's free. It supports multiple accounts for those of you with a ton of alts. However, I don't use it for one reason: it seems to refresh DMs on a fixed schedule, and not on demand. This makes coordinating privately on SLs very difficult.
My twitter client of choice is currently Osfoora. I've tried most others, and keep coming back to it. It's got great multiple account support, has a very handy address book to pick names from in the event you can't remember someone's Twitter ID, and it lets you refresh all aspects of your timeline, including DMs, on command. It costs $2.99 but if you're going to do any heavy RP on your iPhone it's $2.99 that's well spent.
The next application that's critical for Twitter RP is called Boxcar. What Boxcar does is provide near-instant notification of tweets in your timeline via the Apple Push Notification system.
You can setup Boxcar to monitor multiple Twitter accounts... and you can have it sound a different beep when tweets land in different accounts. This is essential if you want to remain on top of things in your timeline. I've become dependent on Boxcar to beep at me even when I'm using my computer - because it simply is that good at what it does.
Boxcar can be configured to pop up a push notification message over the top of whatever you are doing if you choose, so you can see @ tweets or DMs if you choose. This is highly configurable as well, so you can have it display the contents of the tweet, or simply say "you have a tweet", or not display a message at all.
Boxcar can also be a fully fledged Twitter client if you choose to use it as such. It's not as nice as Twitter for iPhone or Osfoora, but it's got that functionality.
Boxcar's price has floated between $2.99 and free. At the moment of this writing it's free, but that may change. It's well worth the $2.99 if you see it at that price - don't wait for a sale for it to be free. It's a great app.
So, for between $3 and $6, you can setup your iPhone or iPod Touch to be a very effective Twitter RP system. For the price of a print magazine, you can tweet from anywhere in near-real time.