I have this issue too, if I disconnect whether intentionally or the connection just drops, ubuntu 10.4 will fail to reconnect unless I unplug and plug it back in. Now, this was not the case with 9.10. I have however found a work arround. if you right click on the network manager applet and "disable mobile broadband" then attempt to connect, it seems to work fine. This would appear to actually be an issue with network manager not tidying up it's DUN connection properly, at least, that's my guess