One iPhone user successfully modified a version of TightVNC to allow him to control his Windows PC from his iPhone.
WebVNC is a modification of the VNC server. Where before the HTTP server in VNC would just serve a Java applet, now it serves an AJAX application that downloads the screen as a set of JPEG images. The application then checks back with the server to see which ones have changed, and reloads them. Input is just another message to the server.
The web-based interface supports Mouse Clicks, Scrolling, Drag/Drop and text entry. Due to the use of Windows-specific functions, the application is only supported on Windows at this time. A downloadable binary is offered along with source code.
One of the reader comments suggests that LogMeIn.com could be another alternative if you set your preferences to "HTML", but in brief testing, I was unable to emulate mouse functions over the web interface.