This hack, still in volatile pre-alpha testing, is based on an onscreen keyboard that the Pre already has—the phone's symbol input panel, for characters that aren't accessible by keyboard shortcuts. By patching the keyboard layout to include "regular" characters, the guys at webOS Internals were able to slap together a simple proof of concept onscreen keyboard, in both portrait and landscape modes, which, as Adrian said before, would make quick, short text input waaay more convenient.
It's too new to recommend installing unless you're versed in the cryptic particulars of Pre homebrew, but given the demand for and simplicity of the hack, it doesn't seem like an easily-installable onscreen keyboard is too far over the horizon. Or, you know, Palm, you could just save us all the trouble. [webOS Internals via PreCentral]