[EricH]

Walkie-talkies are out (Section 3.3, last bullet). I would also assume Bluetooth is out, due to the following (rule shortened for readability):

<R02> ROBOT parts shall not be made from hazardous materials, be unsafe, or cause an unsafe condition. Items specifically PROHIBITED from use on the ROBOT include:
 Shields, curtains, or any other devices or materials designed or used to obstruct or limit the vision of any DRIVERS and/or COACHES and/or interfere their ability to safely control their ROBOT

(emphasis mine)
IIRC, Bluetooth is infrared based. I would assume that you can't use it due to interference risk.

Also see <T22>.

<T22> The only equipment that may be brought on to the field is the OPERATOR CONSOLE, reasonable decorative items, and special clothing and/or equipment required due to a disability. Other items, particularly those intended to provide a competitive advantage for the ROBOCOACH, are prohibited.
Devices used solely for the purpose of planning or tracking strategy of game play are allowed inside the ALLIANCE ZONE, if they meet ALL of the following conditions:
 Do not connect or attach to the OPERATOR CONSOLE
 Do not connect or attach to the FIELD or ARENA
 Do not connect or attach to another alliance member
 Do not communicate with anything or anyone outside of the ALLIANCE ZONE (for example wireless communications must be disabled).
 Do not in any way affect the outcome of a MATCH, other than by allowing team members to plan or track strategy for the purposes of communication of that strategy to other alliance members.

So, communication would be allowed (not disallowed), but it can't use wireless communications. Other means would need to follow the rules as well.