Freedom make bluetooth keyboards that work with at least some Android phones.
I use the i-Connex mini
with my Motorola Defy+ (Android 2.3 but also worked with the older Defy on Android 2.2). It also works fine with my wife's iPhone 4.
I think the key is to see if your phone supports HID (Human Interface Device), or if not, to install a driver that supports it like http://www.teksoftco.com/?section=blueinputandroid.
Sometimes a keyboard is just faster than onscreen input on a phone. I can't speak for iPad since I don't have one.
At work I use a Windows tablet PC (Lenovo X201 Tablet) with Microsoft OneNote to capture handwritten notes and later convert some/all to text. Would love to see something as competent on Android or iPad, but since it needs an 'active' digitizer to get sufficient resolution for accurate recognition, it may need to wait until the next generation of tablets.