I like how people, who i assume to be geared riders, have immediately attacked SSing. I dont know whether its tongue cheek but whatever, who cares.
The way i see it, Singlespeeding makes you think more about how you cycle and definately makes you use your upper body more. Especially your lower back from churning your chosen gear up steep hills. I PERSONALLY find SSing much more involving and rewarding experience. I wouldn't however say it is for everyone, some people simply prefer gears and/or suspension.
My suggestion is go and buy a singlespeed kit for about 20 quid and convert whatever your current XC/Commuting bike is with it. Okay its not ideal like track dropouts are but then you dont have the choice of gears and you have to think about those hills.
Ive never walked where a geared bike could still pedal.
My bike is no weight weeny.
Ive never used the excuse "ive only got one gear", in fact quite the opposite.
The only time innapropriate gearing comes into play is when the riders a wimp.