Yes, I could imagine playing Gears of War 3, or any other similar, graphically-intensive game, on a tablet. However, I wouldn't expect to physically interact the tablet whilst doing so.
To elaborate on the first part, as smart phones and tablets become more complex and powerful, they will begin to encroach and eventually overlap with the processing and graphical capabilities of consoles. (The PowerVR G6200/G6400, let alone NVIDIA's Tegra offerings and the state-of-the-art devices in the research literature, are a testament to this, from a GPU standpoint. From a CPU one, the quad-core ARMv7 Cortex-A15 handily beats out the triple-core IBM Xenon in the Xbox 360 and the Cell processor in the PS3, in terms of MIPS.) As this happens, there will only be a handful of relatively minor reasons, most of which concern how to handle older, potentially out-dated devices, as to why we could not expect to see quality games ported over to these mobile platforms, let alone have studios change focus and solely push their titles for them.
Now, as for actually playing the games, it's easy to imagine a few scenarios for how this could be done for a variety of titles. One that would work well, in general, would be to interface the tablet or phone with a TV, either through an HDMI connection or perhaps wirelessly through something like the Apple TV, and rely on one or more Bluetooth controllers for input. In this instance, the device is functioning like a console; however, once you're done playing, you can just grab it, take it with you, and revert back to using it for a multitude of other purposes.