Sorting, as it exists, was not originally part of RTM. It was added on 8/1/07, and those three sort options (priority, date, alphabetical) are better than nothing, but...

These boards are littered with threads by people suggesting different sorting schemes. A very non-comprehensive list follows, including several that ask for just what you desire...

http://www.rememberthemilk.com/forums/ideas/279/
http://www.rememberthemilk.com/forums/ideas/552/
http://www.rememberthemilk.com/forums/ideas/783/
http://www.rememberthemilk.com/forums/ideas/1448/
http://www.rememberthemilk.com/forums/ideas/1547/
http://www.rememberthemilk.com/forums/ideas/1891/
http://www.rememberthemilk.com/forums/ideas/1910/
http://www.rememberthemilk.com/forums/ideas/2243/
http://www.rememberthemilk.com/forums/ideas/2514/
http://www.rememberthemilk.com/forums/ideas/2854/
http://www.rememberthemilk.com/forums/ideas/2856/
http://www.rememberthemilk.com/forums/ideas/2879/
http://www.rememberthemilk.com/forums/ideas/3145/
http://www.rememberthemilk.com/forums/ideas/3158/
http://www.rememberthemilk.com/forums/ideas/3206/
http://www.rememberthemilk.com/forums/ideas/3233/
http://www.rememberthemilk.com/forums/ideas/3742/
http://www.rememberthemilk.com/forums/ideas/3835/
http://www.rememberthemilk.com/forums/ideas/3989/
http://www.rememberthemilk.com/forums/help/4043/
http://www.rememberthemilk.com/forums/ideas/4472/
http://www.rememberthemilk.com/forums/ideas/5062/

So when you hear cranky ol' timers lament that the UI of the RTM site has lain dormant while other things have been attended to - GMail, iPhone, &c. - this is exactly the sort of thing we're talking about. I really hope some basic things, like per-list, multi-variate, customizable sorting, are addressed before Bob turns his attention to Outlook syncing, for example.

* The idea that smartlists have access to an operator that would specify sorting is one of the many great ideas found in the threads above. I think that would help make RTM's best feature even better.