It's a flawed way to calculate online users as already mentioned some people have multiple accounts. Having said that, I think the vast majority would only have one, so whilst it would push it up a bit, I don't think it would massively inflate the numbers.

Say for example there are 1 million hardcore users who actually know how to, and also bother to create three accounts each, then that would only be an additional 2 million accounts. I just made those numbers up as an example.

SCE Poland said that about 50% were online in Poland, and that's a country with relatively low broadband penetration, we can also assume that not many non-Polish people would bother to create an additional Polish account.

For countries with far higher rates of broadband penetration such as NA and Western Europe I would expect it to be much higher than 50%. 75% would seem a reasonable number.

I have only a single US account. My two friends, who each play on their PS3 regularly, together have 3 accounts.