Actually you don't even have to separate it with spaces. I used the command word 'Work' (case sensitive btw) and accidentally typed "Working over today, be home soon!" and it activated the speaker.

You can use that Case sensitive thing as an aid. I've used 'iphone' as a command for a long time. It's auto corrected to 'iPhone' like apple wants it but I force it back to all lower case and it activates a command. Example "Sry, iphone signal is terrible here." or "Dang iphone battery dying" or whatever you like.

Hope this helps!

P.S. I do agree however that invisible commands like the symbian phones would be ideal.