GPS has been used in cruise missiles since the mid 90s. Bombs really don't need any great accuracy.

For anyone reading this thread out of interest, I should clarify that the type of differential GPS I was talking about has a relative accuracy of less than a centimetre. The absolute accuracy is much larger, and depends on how long you record data for and how it is processed, but is probably a few metres.

So you can tell how far apart two things are, and the direction between them, with much greater accuracy than the actual position of either of them.