What could happen if I plug into my Arduino an AC adapter instead of a DC one?
Assuming what you have is actually an adapter that outputs AC, this is what will happen:


The Arduino will fry: It does not have a bridge rectifier on board. The reversed half-cycle will destroy the board.
The motors won't work either, because, as you noted, they are DC motors.


However, as an AC output does not have a "center pin is positive", given that they are AC, it is quite likely that you have a AC to DC adapter. You will most probably just suffer from fried motor syndrome - 4.5 Volt motors can smell quite delicious when powered by 12 Volts - especially after the sizzling stops. 



Other caveats: If you are planning to drive those 4.5 Volt motors directly off GPIO pins on the Arduino board, your Arduino will be toast anyway, for these reasons:


The GPIO pins should not be used to drive more than approximately 20-30 mA current per pin (the official rating is 40 mA per GPIO, absolute maximum), else overheating and thermal death ensues. Those motors need to be really tiny ones that operate at say 10 milliamperes or less - even tiny pager motors need more, though specialty micro-motors rated below 10 mA do exist. 
The Arduino pins cannot handle the back-EMF the motors will generate, so the microcontroller will die  ... permanently. 
The voltage regulator on the Arduino will heat up horribly and may either thermal-shutdown or be destroyed if the motor's current drawn is significantly more than around 150 mA, even if you drive the motors directly from the +5V pin on the Arduino, ignoring the GPIO pins. The on-board regulator is a linear regulator, so heat generated is (12 - 5) x current. Any non-trivial current, i.e. the motor's load, will generate a lot of heat at the regulator.




Edit: In response to comment providing image of adapter:



Notice the brightened line of text, which specifies the output of the adapter. The symbol circled in red indicates DC output. 

The tilde ~ symbol directly above it indicates AC, i.e. the input is AC, hence the term AC adapter.

If the output had been AC, the same tilde symbol would be used in the output line as well.