I've been taking Japanese enough to read hiragana now, so I typed both variations (in hiragana) into babelfish and got the exact same result ("The time stop". lol engrish). Anyway, the kana "wo" can also be pronounced "o", which leads me to believe the correct way to romanize it is "Tokiyo Tomare" no matter whether the "yo" or "wo" kana is used.
After a bit of searching around, I've found that "yo" is mostly used emphasize a command. So "Tokiyo Tomare" would become roughly "Stop, time!". However, it's supposed to be used at the end of a sentence, I think, so I'm not sure.