Among the crowded field of cloud sharing and file synchronization services, there are plenty of interesting options (we've been partial to Dropbox around here, but check around for more), but when you add 'must work well with iPhone' to the requirements list, the choices narrow a bit. I've been testing out two business-focused options in Readdle Docs and box.net, and two other iPhone-friendly options (ZumoDrive and Drop.io) have just poked above the horizon.
Readdle Docs may be on the pricey side for the App Store at $9.99US, but there's a lot of functionality bundled into that ten bucks. In addition to turning your iPhone or iPod touch into a WebDAV file server -- accessible easily and securely in the Finder, Windows Explorer, or via clients like Transmit -- Readdle lets you access hosted storage on your MobileMe account or other WebDAV services.
Once you get the files onto your device, it's easy to view all iPhone-friendly formats, or forward documents and images by email. I'm particularly fond of the full-screen PDF and PPT viewers in the app. Of course, it's no Documents to Go, but it's a good start (and for those waiting eagerly for D2G to arrive on the iPhone, word from DataViz is that the app is on track for a Q2 release).
Update: DistortedLoop points to veteran Palm developer QuickOffice and the $9.99 Mobile Files Pro product, which offers iDisk connectivity and also allows editing of Excel files. I hadn't heard of it before and we'll definitely be checking it out for a future review.
While you can use Readdle with iDisk or any WebDAV-enabled storage service, you also get a bonus account on the Readdle servers with 512 MB of storage (yes, not much, but it's handy). Upgrades to the Readdle storage start at $5/month for 2 GB. On the other hand, if you need the cloud access but not the onboard server, you can save $2 and get Readdle's OneDisk product for $7.99. Per Michael's question in the comments, the Readdle team does have OneDisk working with JungleDisk and Amazon S3, and it should work in the same fashion with Docs.
Though box.net is focused on enterprise users, it's plenty functional for personal use; with a free 1GB storage account that permits 5 collaboration folders and files up to 25 MB in size, you've got some room to maneuver (paid plans start at $7.95/mo). The companion iPhone app, also free, lets you upload images from your photo library or camera, view a list of recently modified files, and open up any of your files for viewing (unfortunately the new box.net webdoc format reads in as raw HTML, which is a drag). You can also forward sharing links to your colleagues directly from the iPhone app, which can be extremely helpful in the midst of a collaborative project.
The box.net app is more streamlined than Readdle Docs, but offers a lot less of the bidirectional functionality, and the viewer isn't fullscreen or landscape enabled; it also doesn't appear to cache large documents locally as Readdle does, and it's crashed on me once or twice while downloading large PDFs. Still, if you're already using box.net or you don't have a WebDAV-enabled storage option for use with Readdle, it's a fine app to try.
Details of the two new kids on the block, in the 2nd half of the post.