I have tethered (PC-Suite mode) a few times before with xubuntu jackalope (9.04) without any issues. Though at times I have to reconnect to wireless broadband twice for it to work. This is using the networkmanager.

Could it happen to be issues with signal? have you also tried forcing to connect to wireless broadband by setting N900 in 3G mode? I know with autodisconnect it will turn off 3G mode when you're not connect to wireless broadband or you're idling beyond the set time frame. This option is not enabled by default but I have noticed sometimes switching from 2.5G to 3G takes sometime and it may case ICD (Internet Connection Daemon) to think that wireless broadband is not available cancelling the wireless broadband access.

If you force it to 3G mode with 2G/3G switcher program you maybe able to get internet access from there.