When I was diagnosed in the USA, I learned all about blood sugar in mg/dl. I was taught that a low blood sugar was anything under 70 mg/dl (a nice round number).
I moved to Hungary and after spending hours converting my blood sugar logs from mg/dl to mmol/L before each doctor's appointment, I finally decided to switch to using mmol/L in my daily life last year. I got used to the new measurements pretty quickly (though I still do sometimes convert the numbers). I was treating lows as anything below 3.9 mmol/L (since 70/18.05=3.87).
At one point my new CDE told me that lows are anything below 3.5 mmol/L (which is 63.17 mg/dl). Magically after that I started not feeling low at 3.7 :-) So I stopped treating the lows unless they are below 3.5. My endo still tries to keep my blood sugars above 4, but considers below 3.5 as the real lows.
But I was wondering about the rest of you who measure in mmol/L -- what is your cut-off for a low blood sugar?