Is there a camera lens adapter that'll allow lenses for a Canon t3i Rebel to work on a Canon Mark 2?

All the answers are correct, but they are still missing an important point. The EF-S lenses have a smaller image circle than the "full frame" EF lenses. In other words, the EF-S lenses are designed to "cover" the sensor size of the smaller sensor in the crop sensor cameras. The full frame cameras, such as the 5D models, have a larger sensor and even if you could somehow mount the EF-S lens, the image produced would not fully cover the larger sensor in the 5D. Your image would be a circle with black corners. So it is not just a matter of physically making the EF-S lens fit the full frame 5D camera, ... there would be problems with the image produced. Using the EF lenses on the crop sensor cameras is just the opposite, there is no problem with that because the EF lens more than covers the smaller sensor in the crop sensor camera.

I always tell people to NOT invest in the EF-S lenses because some day they may upgrade to a full frame camera and the EF-S lenses will be worthless.

No.

Canon took advantage of the smaller sensor to design a series of smaller and lighter lenses which, unfortunately, protrude too far into the camera body to be used on full-frame models.

An adapter that would allow the lens' rear element to clear the full-frame body's mirror wouldn't permit infinity focus.

Canon makes two lines of lenses for their DSLRs.

EF Mount - This mount is for their 35mm-format lenses which are optimized for their full-frame DSLRs. You can use these lenses on Canon's APS-C DSLRs, but the focal length will increase by 1.6x. So an EF 50mm f/1.8 lens on an APS-C body will have an equivalent angle of view as an 80mm lens on a full-frame camera. However, a 50mm on an APS-C body will not have the same depth of field as an 80mm. Multiply the aperture by the 1.6x crop factor to get what you'd need on a full-frame camera to obtain the same amount of dof. Conversely, just divide the aperture used on a full-frame camera by the crop factor to get what you'd need to use on an APS-C body to obtain the same depth of field.

EF-S - Lenses with the EF-S mount are optimized for APS-C bodies. Due to their design, it is not possible to mount these lenses onto any Canon full-frame body.

None.

The EF-S mount lens will mount but it's extended rear end will get in the way of the full-frame's reflex mirror which is guaranteed costly repair job. Anyways, if you can afford to buy a Mark II, you should be able to afford EF-mount lenses with the L spec even so what's with this question?