They can access the contents of the transmission as they are transmitted if they can get an appropriate type of radio where there is signal. For wifi or wimax, it wouldn't be very directional and thus wouldn't be too hard to pick up, even with a fairly directional antenna. For microwave, it's pretty direct line of site, but would still be able to be picked up if they can get in-between the transmitters.
If the data is encrypted in transit (which it should be) though, then the damage from this may be rather limited. Most cable and fiber optic connections use passive fiber splitting and aren't significantly more secure than wireless since the signal from the central office is shared with numerous customers and only a simple filter in the cable modem keeps the traffic from being delivered unless they are employing encryption on the transfer.
The big thing would be to ask your ISP if they use encryption between your location and their main office. Obviously if you are using your own nested encryption (such as HTTPS) then you are protected by that regardless. A VPN would also protect you across the link if they don't encrypt the link.