FIRSTLY: I make no guaranties this will work, comes with no warranties and will not be responsible if it blows up your phone. However like all jailbreak stuff, there has never been a brick that DFU mode can't recover.
As I'm sure every iPhone user knows, there's a roaming toggle in Settings. However, this relies on Carrier support for it to work.
Now, as I'm doubly sure every Three customer in AU knows, the toggle only applies to "International Roaming", and when you leave Three's 3G zone and roam in to Telstra's nextG, 50c/meg fees apply. With a bunch of data hungry stuff running on your phone, this is potentially dangerous to your wallet.
Enter Romophobic, this will in theory work on all carries, but if your carrier supports the official toggle, use that.
Please note, I have not added the toggle to BossPrefs yet, so the script will be auto started with the phone, check back on the weekend for an update for featuring a toggle. I want some feed back before going any further.