Tap in any of the text entry fields. If they are editable then the keyboard will pop up.
There are two reasons (I can think of) that might prevent this. One, the website is using Java, Flash, or some other unsupported plugin for the login screen. I know one or two that actually have their login's as pop-up windows. Those might work if you turn off the pop-up blocking in Settings > Safari. Or they might not.
The other is if you have a bluetooth keyboard paired and turned on. The iPad will detect this and the keyboard will not come up. You can force the keyboard up with the eject key (if you have one), tapping the small keyboard icon at the bottom right, or turning off the bluetooth keyboard (or shutting of bluetooth on the iPad).