Sony has revealed that it has no plans to release a wireless adapter for the 20GB PS3.
With HMDI now present in both versions of the PS3, one of the other major differences is the built-in wireless adapter found only in the 60GB model. In a Q&A posted on the official Japanese PS3 website, the question of upgrading the 20GB to make it wireless is asked. The answer: "Not currently".
Obviously the 20GB PS3 does have an Ethernet port so if you want you could plug in a wireless bridge of some kind and hey presto your 20GB PS3 is wireless. There's also the possibility that Sony will release a wireless adapter by way of USB dongle.
Obviously this will only become a problem for European gamers if Sony decides to release the 20GB version in Europe. Back when the console was scheduled to arrive this year, Sony was remaining tight-lipped over the release of the 20GB model.