Apple's Keynote, Pages and Numbers apps on the iPad have long been not only useful apps, but symbolically important apps. The apps are useful because they all do a good job of creating documents. Additionally, these apps demonstrate that the iPad can be used to create work-related documents. Especially when the iPad was originally introduced, these apps proved that the iPad was not just a toy or a content consumption device. These apps have received good reviews and I use all of them on my iPad from time to time. Indeed, there have been several times when I have found it easier to create a presentation in Keynote on my iPad than to create a presentation running PowerPoint or Keynote on my computer.
Yesterday, Apple updated all three of these apps so that they now run on the iPhone. (They work on the iPhone 4, iPhone 3GS, and the third- and fourth-generation iPod touch.) These are universal apps so if you already have them for your iPad, the same apps will now run on an iPhone. These three apps can handle not only documents created in Keynote, Pages or Numbers on the Mac or iPad, but can also read and edit Microsoft Office documents (PowerPoint, Word and Excel).
These apps are useful on an iPad, but is the same true on the iPhone with its much smaller screen? Because the apps are brand new it is a little early to tell. I can imagine situations in which the Keynote app would be useful. Creating a presentation on the iPhone seems cumbersome, but if you create a Keynote presentation on the computer or iPad I can see it being useful to (1) make some quick edits on the iPhone, (2) review your presentation on the iPhone as you rehearse in an environment (such as on a plane) where you don't want to use your computer or even an iPad, and (3) run your presentation by connecting a cable from the iPhone to a projector or monitor. Note that Keynote is, I believe, the first Apple app that runs only in landscape mode.
As for the Numbers and Pages apps, I find it hard to envision spending much time using these apps on the iPhone. Pages is a star on the iPad when you want to create a document with a nice layout, but the iPhone screen is too small to really appreciate a layout on a document. I would prefer to use a program like Quickoffice or Documents to Go, apps that focus on making the text easy to read without an undue emphasis on preserving full-page formatting. And any Numbers spreadsheet with more than a few columns and rows is going to be difficult to work with on the iPhone screen. Curiously, Numbers only works in the portrait mode, even though many spreadsheets seem more suited for a landscape view.
Although I am currently skeptical that I will use Pages or Numbers on the iPhone very often, I must admit that when iMovie was released for the iPhone, I doubted that the app would be very useful on the small screen. My opinion has changed over time becasue I have frequently recorded some videos on the iPhone and then used iMovie to quickly turn them into something more professional by merging and editing clips and by adding some titles and a soundtrack. Perhaps over time I will find circumstances in which creating or editing documents in Pages or Numbers makes sense on an iPhone.
It is certainly impressive that Apple found a way to make these sophisticated apps usable on the small iPhone screen. For example, as Dan Moren of Macworld notes: "To ease working with the smaller screens, a Smart Zoom feature for Pages and Numbers automatically follows the cursor while you’re editing text or data, then zooms back out to give you an overview when you’re done." If nothing else, having these apps installed on your iPhone will make it easier to view documents created in Keynote, Pages or Numbers on an iPad or desktop when someone e-mails them to you.
Click here to get Keynote ($9.99): 
Click here to get Pages ($9.99):
Click here to get Numbers ($9.99):
