For wireless comms between Arduinos, take a look at the nRF24L01 modules. You can get them from china for £1.32 each on eBay, or about 2.50-£3.00 from UK.
Then look for RF24 and RF24Network libraries by maniacbug.
I've started playing with these myself and they are nice little devices. If you don't need to transmit far, less than 10m, the those cheap low powered ones will do. If you need more range then you get higher powered units for about £7.50 each.
They are a much cheaper alternative to XBee, Zignee or Wirelss Shield. They also work in 2.4Ghz so you can use any normal wifi type antennas with the higher powered ones.