I think the value of 20 seconds is arbitrary, but there's a good reason why they don't lower it any further. With a protocol like TCP that guarantees reliable delivery, sometimes packets get resent to the server. If the server receives this resent packet after the connection has been closed and a new connection has been opened, it is possible that the server will accept this packet. As such, as part of closing a TCP connection, the connection sits in a TIME-WAIT state for 2*MSL seconds (MSL = maximum segment lifetime).

If you decrease the amount of time a connection sits in a TIME-WAIT state, you will increase the probability that a new connection could receive a packet from a previous connection. Given the latency of mobile networks, I would expect the probability of receiving a packet from a previous connection to be even higher.

If you're interested, the beginning of the following paper provides a great overview of the problem: http://www.isi.edu/touch/pubs/infocomm99/infocomm99-web/