Are you hoping to use it for running text? Our corp site is Typekit, and it works just fine with Safari 5 (we're an all-Mac shop). But we're not trying to set body text in Minion or anything; we use a neutral sans for the body, and FF DIN for heds and subheds.
We're not aware of any issues in Safari 5 - could you drop an email to support@typekit.com with an example URL showing the problems so we can look into it?