I have noticed that a lot of RF IC's use differential input/outputs, is this "just" to cancel common-mode noise on the lines (in the circuit), or is there another reason for this?
Before CAT-5 networked PCs were often connected via coax cable like RG-50. This was single-ended. Today we use twisted-wire, and do so at much higher frequencies. Twisted-wire is much cheaper than coax, and offers the same EMI-immunity. Twisted-wire asks for a balanced signal, i.e. differential.
The reason why twisted-wire requires a balanced signal: if interference is picked up by both wires, it will have the same amplitude and sign for both, and will be canceled out when both lines are subtracted from each other.
If a single-ended wire (where one of the conductors is ground) would pick up interference it would remain on the signal wire and be neutralized on the ground wire.
This makes differential signaling highly immune to noise and allows for low level signals, like for instance in LVDS (Low Voltage Differential Signaling), which allows for low supply voltages down to 2.5V.