I thought I had it at least working.

After logging in with a second acocunt on the Macbook Pro, it would not even initialise. Then back on my normal account, it would no longer connect either.
I started getting console errors:

7/09/09 9:35:42 AM com.apple.kextd[10] Can't open CFBundle for /System/Library/Extensions/IOSerialFamily.kext/Contents/PlugIns/AppleWWANSupport1.kext.
7/09/09 9:35:42 AM com.apple.kextd[10] Kext id com.apple.driver.AppleWWANSupport1 not found; removing personalities.

The strange thing is that the original account I got it working on was using AppleWWANSupport.kext NOT AppleWWANSupport1.kext.

I’m really not in the mood for reinstalling the whole O/S, so the way I got this working again was to:

1. ensure I had the AppleWWANSupport1.kext and AppleWWANSupport.kext folders back in place in /System/Library/Extensions/IOSerialFamily.kext/Contents/PlugIns/ (thanks Timemachine backup!)
2. remove the “Novatel Wireless HSDPA Modem” from the Network services list
3. and as others have said in the thread – Reboot with the card plugged in.
4. I then logged in with my second account (that never worked with the XU870) and added the new network service, choosing the “Novatel Wireless HSDPA Modem“
5. Under advanced settings I then removed the default telstra.datapack that was added putting the 3netaccess APN in for the ISP I use

I had the same problem. I eventually deleted all the Novatel files in: /System/Library/Extension/ as follows:
rm NovatelMerlin.kext NovatelWirelessFilter.kext NovatelWireless3G.kext

I rebooted – and everything worked fine.

Hope that helps.