I have a bit of extra information for anyone following this thread.

1. The information in /var/log/syslog is to be treated with caution. As can be clearly seen above, whilst the info in the syslog suggests ttyACM0 is the correct device, in fact it turned out to be ttyACM0. I've noticed a few other strange messages, but I don't want to confuse the issue.

2. I can only use wvdial as root or in conjunction with the sudo command. Oddly, however, I am not required to enter a password after the first time.

3. It's not clear whether or not this is related to the modem problem, but my evolution e-mail has ceased working correctly. It starts in off-line mode and the option to switch to on-line is disabled. I'm going to trawl the forums for this issue, now. If no solution presents itself, I'll have to sart a new thread, I suppose.