I can see the bold text in Opera and FF on Mac OSX the same as in Safari. I did read the following about using the fonts:
The Font API provides the regular version of the requested fonts by default. To request other styles or weights, append a colon (:) to the name of the font, followed by a list of styles or weights separated by commas (,).
To find out which styles and weights are available for a given font, see the font’s listing in the Google Font Directory.
For each style you request, you can give either the full name or an abbreviation; for weights, you can alternatively specify a numerical weight:
italic italic or i
bold bold or b or a numerical weight such as 700
bold italic bolditalic or bi
For example, to request Cantarell italic and Droid Serif bold, you could use any of the following URLs: