High freq. response of op amp using Miller
To be able to find the unity-gain frequency you can use this trick that will work for this practical circuit.  







simulate this circuit &ndash; Schematic created using CircuitLab

Hence \$V_X = \frac{\frac{1}{s C1 (1+A)}}{R1+\frac{1}{s C1 (1+A)}}\$

And because we are interested only in magnitude we can write: 

$$V_X = \frac{1}{\sqrt{1+(\frac{F}{F_H})^2}}$$  

Where the \$F\$ - is a signal frequency and \$F_H\$ - is -3dB frequency.  

And finally we have: 

$$\frac{1}{\sqrt{1+(\frac{F}{F_H})^2}} * |A| = 1  $$ 

All you need is to solve for \$F\$

Hence the unity-gain frequency is 

$$F_T = \frac{1}{2 \pi R_1 C_1} * \frac{\sqrt{A^2 -1}}{|A|+1}$$

Of-course the unity-gain frequency can also be find from transfer function directly.