The iPad's iOS doesn't support Zipped files, which many attachments are converted into by their mail server to save on bandwidth and mail archive space. Fortunately, a great little 99-cent app called ZipThat for the iPad (but not for the iPhone or iPod Touch, unfortunately) unzips mail attachments and lets you open the unzipped file in a program such as Quickoffice or Documents to Go. Basically, it uses the Open In facility to get the zipped file from Mail, then checks the file registry to see what other apps can open the unzipped versions, in a double hand-off.
If you use cloud storage services, you'll find they use essentially the same mechanism to share files. That means an app like Quickoffice or Documents to Go will have a facility to check these services and download any files you choose into their sandbox, so they can work on them. Of course, downloading a PowerPoint presentation this way within Quickoffice (for example) makes it available only to Quickoffice, so you need to be careful about version control. Get in the habit of uploading changed versions quickly, so the next app you access the file from has the latest version.
There is a fourth way to get some data onto an iPad: You can drag PDF files into iTunes' iBooks pane on your PC or Mac and have them synced to the iBooks app on your iPad for viewing there. (You need to have iBooks 1.1 or later installed on the iPad and have synced the iPad at least once after installing it for iTunes to display the iBooks pane.) Too bad you can't do the same for other types of data, such as Office files for use in Quickbooks or Documents to Go.
The iPad input and output issue
The move to smartphones and slates brings with it a big I/O gap: The peripherals we use every day are much larger and thus easier to use than what a portable device has. The iPad's screen and virtual keyboard are plenty big enough for casual work, but not for all-day use or more complex applications. So the ability to tap into external peripherals is key to broad business usage.
Keyboards. I find it very easy to touch-type on the iPad's virtual keyboard -- much easier than on the iPhone or iPod Touch. One tip, though: If you type fast, as I do, disable the Zoom feature in the Settings app's Accessibility section. By default, tapping the screen with three fingers simultaneously causes it to zoom in (a feature for the visually impaired), and having that happen unexpectedly when your fingers strike within microseconds of each other as you type can be very disconcerting.
If you get the $50 Apple iPad case (which I strongly recommend), you'll find it not only protects the device but makes a perfectly angled "keyboard tray" for the iPad, so it's even easier to type on a desktop or other work surface. The typing and viewing angle are just right for most people, both on a work surface and on your lap.
For text input, Apple offers two external options. You can use the Bluetooth-based $69 Apple Wireless Keyboard (or any compatible one, such as those sold for use with Macs), or you can use the $89 Apple iPad Keyboard Dock. Neither is quite right, though the Apple Wireless Keyboard is a bit better. (You can also get some USB keyboards to work through Apple's $30 iPad Camera Connection kit, though Apple won't support such use and could disable it in future iOS updates.)
The issues with the keyboard dock are that you can't also connect the iPad into a larger screen (as you can if you use a Bluetooth keyboard), and your iPad won't fit in the dock if you use the Apple (or any) iPad case; removing the iPad from the case is a bit of a chore, due to its nonslip surface and form-fitting design. But you do get keys or iPad-specific functions, such as jumping to the Home screen and doing a Spotlight search.
The issue with the wireless keyboard is that it lacks those dedicated iPad keys. And there are no shortcuts to access these features, either, in the iOS. Plus, the iPad's various office productivity apps mysteriously don't use keyboard shortcuts for formatting such as boldface and italics, nor do they have shortcuts for paging up and down within a document (Apple has no Page Up, Page Down, Home, or End keys on its iPad-compatible keyboards, due to the company's historic disdain for non-mouse-based navigation). You have to use on-screen controls instead.