Better late than never.
Had the same problem, figured it out. It takes a couple steps, hopefully they'll address this in Yosemite.
NOTE:
These instructions are for a MacBook Air running OS X 10.9.4; if you're using an iMac/MacMini/MacPro, it may be slightly different depending on what KB you use, and if you're using a BLUETOOTH KEYBOARD, doing this will DISCONNECT THAT TOO, so this is really only for devices with either built-in keyboards, or wired ones.
Use this to disconnect when bluetooth is inhibiting your use of the built-in trackpad... * * * O N L Y ! ! ! * * *
So here's what you do:
1. You need to have the "All controls" radio-button option selected in System Preferences>Keyboard>Shortcuts.
(If you have not selected "Use all F-Keys as standard function keys" under System Preferences>Keyboard>Keyboard, you can toggle this without accessing settings by holding Function and Control keys, and pressing F7.)
Thus, when you hit the TAB key, it'll iteratively move the focus from one button to the next.
2. Use Command Space, (or whatever keybinding you may have assigned to activate Spotlight) and type "Bluetooth" then use the down-arrow to move down to System Preferences>Bluetooth and press ENTER. Make sure you use the arrow keys to move down to the System Preferences>Bluetooth in Spotlight, and NOT Bluetooth File Exchange, as that's NOT where you need to be. Basically, however you get there, open System Preferences>Bluetooth.
3. Press tab, highlighting the Turn Bluetooth Off button, and press the spacebar to activate the control. If it comes up already highlighted, just press space to turn it off. Then your built-in trackpad should work again, and as the Brits say, "Bob's your Uncle".
Optionally, just carry the computer far enough away from the mouse for it to lose contact, then turn bluetooth off using the trackpad.
There should be a keyboard shortcut to do this, and it's a glaring oversight that there's not one; I have sent a suggestion to Apple that this be included in Yosemite, so hopefully they'll listen and add that if they haven't already.
I have personally tested this method, so I know it works as a minimum on 10.9.4 on a MacBook Air, (2013 edition, I think) and no external software is needed.
Finally, one other option is just remember to shut the mouse off (there's a switch on the bottom of the Magic Mouse, not sure about others,) which also conserves its battery and restores the built-in trackpad without these steps.
These are instructions for turning your device's bluetooth transceiver off only, not picking and choosing which devices are connected. That can in fact be done, but requires un-pairing, which will require RE-PAIRING later if you want to use the device again. The steps for that are similar, but not what I'm writing about. For that, instead of step 3, you tab over to the device list, select the device, and hit space, at which point it should give you more or less this same warning as I'm giving here, so you'd probably want to do this if you're using a non-portable Mac, but that means you won't have a working pointing device anymore unless you have another one ready to use in its place... which if you did, you probably wouldn't need this, but I digress.
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Good luck, and thanks for reading!
