Alright, I figured this out, so in case this helps anyone...
I'd previously (in fact it was the first thing I'd done since downloading RC5) followed another posts directions to create an additional field for my tickets called "company".
My issue was being caused by the silent failure of the ticket creation function, which failed because there was no "company" variable being passed to it when email was being processed.
adding a dummy var to class.mailfetch.php at around line 230-ish resulted in proper operation.
So to summarise, any additional fields entered must also be catered for in this file, so email is properly routed.
In my example, because I want to capture the company a user is emailling from I've used something like:
$fromco = explode("@",$var);
$var=$fromco;
Resulting in my company field being populated with "microsoft.com".
Hey I'm a web developer, domain name is good enough for now at 2!
I sincerely hope this saves 6 hours of someone elses life!