At this point, you might tempted to say something like $%&!@#$..., but hold that thought. Canon actually gave us a very nice little feature that makes the Tv mode a bit like a hybrid Tv/Av mode. Here are the steps:
Step 1: Setup the Shot
Spin the mode dial to put the camera into Tv mode, choose the focus mode, set the zoom... In other words, frame the picture as if you were going to take the shot.
Step 2: Meter the Shot
Put the focus point on target and press the AE Lock/FE Lock button instead of the shutter button (it's the little round asterisk * looking button on the top right corner of the camera while looking at the back). Now the cool part starts.
One of the things you've done here is tell the camera to take a meter reading and, if you look quickly before it disappears, you'll see a meter popup on the display that shows a dual shutter/aperture scale.
If it disappeared before you got a good look, don't worry. You'll see it again in step 3. As a side note, the asterisk next to the shutter speed on the display indicates you've locked the exposure.
Step 3: Rotate the Control Ring
When you rotate the control ring (the rotating ring outside of the FUNC. SET button extravaganza) the G10 will display the dual meter and allow you to set the either shutter or aperture to your liking before you shoot. Since you've locked the exposure, the G10 will move the shutter and aperture settings together in its attempt to maintain correct exposure.
Step 4: Shoot
Re-target the subject and press the shutter release.
OK, I know it would have been nice to have shutter settings longer than 1 second while in Av mode. You have to admit though, that changing the shutter and aperture while maintaining the correct exposure is a nice feature.
Two Parting Comments
First, you can intentionally under or over expose things if you like by setting the shutter far to high or too low prior to step 2 above. When you do, the camera will let you move the shutter speed beyond the area of correct exposure. Play with it and you'll see what I mean. In reality though, it would probably be easier to just use the exposure compensation dial instead.
Second, you can access the same dual meter gizmo if you press AE/FE Lock when in Av mode or Manual mode, however, the behavior is a bit different in each. In Av mode, you can't move outside of the correct exposure bounds. In M mode, the aperture and shutter are never really locked, but the dual meter does appear.