I suppose this fall backs to active RFID type of communication issues and, maybe 1-2 years ago, I read an article on the subject (sadly I do not remember the reference now). Researchers tried to authenticate under the following condition:
- The victim bearing the genuine RFID token was in some crowded public place (subway, ...),
- Attacker A goes near the victim (no need to steal the token), he bears an RFID tranceiver linked to a mobile phone,
- Attacker B is near the door to open, having also a mobile phone and an RFID tranceiver.
Here is a quick representation of the communication path established between the RFID token and the RFID lock:
Genuine RFID token <--> RFID tranceiver <--> Mobile phone A <--> Mobile phone B <--> RFID tranceiver <--> RFID lock
Thanks to this system, the researchers manage to extend the RFID range to unlimited. However, the issue they encountered was, as you mentioned, timing issue making this attack practically not doable with current technology (ie. up to 3.5G), however they emitted some worries on the subject with technology evolution which would fatally come up with this timing issue.
So, to answer your question, technically the attack is doable, but not practical yet. However, if I were to wear such a key and would have such safety concern, we see more and more faraday pouches designed to prevent such issue (some are even integrated to bags and jacket pockets, a discussion on the subject is here).