The following text is set in Helvetica weights between 100 and 900, on 100-300 it appears in italic!?! Seems to be a similar issue reported in this thread. Seems to be fine in Chrome 7, Safari 5 and Firefox/Minefield 4.0 on the same machine. Opera 10.63 is OK-ish - whilst it doesn't render the thinner type in italic, but it doesn't look any different from the 400-500 weights either. Other fonts such as Arial are fine. Also, on the browsers which render the weight correctly, I'm a tad surprised that weights 100-300 come out marginally wider than the same text at 400-500...
The cat sat on the mat. The quick brown fox jumped over the lazy dog. Weight=100
The cat sat on the mat. The quick brown fox jumped over the lazy dog. Weight=200
The cat sat on the mat. The quick brown fox jumped over the lazy dog. Weight=300
The cat sat on the mat. The quick brown fox jumped over the lazy dog. Weight=400
The cat sat on the mat. The quick brown fox jumped over the lazy dog. Weight=500
The cat sat on the mat. The quick brown fox jumped over the lazy dog. Weight=600
The cat sat on the mat. The quick brown fox jumped over the lazy dog. Weight=700
The cat sat on the mat. The quick brown fox jumped over the lazy dog. Weight=800
The cat sat on the mat. The quick brown fox jumped over the lazy dog. Weight=900