It might be perfectly clear to most of you (then sorry for this message), but it wasn't to me, because I feel this is different on ARM7TDMI MCUs...
Just to save some debugging and googling: In the case you experience an unexpected bus fault when accessing any peripheral registers (at least on LPC17xx), just make sure you didn't forget to switch the peripheral power bits (PCONP register). As stated in the Users Manual:
Important: valid read from a peripheral register and valid write to a peripheral register is possible only if that peripheral is enabled in the PCONP register!
Might be helpful for someone.