I'm not sure if this tip has been posted before, but I just got an email with it from Mike Hayashi. I'll post it verbatum:

You can trick your Sony A3 or A2 dss receiver to be controlled by both ir and rf signals.

* Make sure you're in rf mode first.
* Press menu, 9-setup, 1-remote.
* Select "remote codes" which will bring up a list of tv codes, etc.
* Now select "tv codes" to bring up the list of tv codes.
Now press exit.

This should leave your dss receiver in a mode where it responds to both ir and rf.

There are two problems with this though. If you use the RF remote in the same room sometimes the dss receiver will respond to both rf and ir signals as if you pressed a button twice. But most of the time that doesn't seem to be a problem very often. Also, if you have another similar dss receiver, you can't do the trick on that since it seems to default to the same rf security code (001) so both rf remotes end up controlling both receivers.