The term 'hacker' is perhaps not so accurate, but more and more forums are reporting stories of people whose PSN password suddenly changed and that with their credit card information, stored in the account, games are purchased by someone else.
With the numerous BETAs and codes which allow you to get additional content, there are a lot of people seeking access to these. They are sometimes fooled by others under false pretenses, and only their login and password to enter PSN is needed. Below is an example of someone who is trying to obtain someone's password to enter through the Home BETA.
People sometimes offer to 'swap' PSN games, however, if you have each other's account information you can download each other's games... this is not only illegal, but secondly someone can change your password so you no longer can access the account. Worse yet, others will be able to access your credit card information.
Sony nor their official developers will ever ask your password and login for PSN as in the user conditions it's clearly prohibited, but because it happens often enough this is still a necessary warning.
Even if people know you on your e-mail address connected to your PSN account, make sure you have a password with a mix of numbers and letters, and not something like '1234 'or' password'. You'd be amazed by how many people are using a password from laziness, and then find out that their password has been changed by another and that there are all sorts of purchases made on PSN.
If you think your account is compromised, make sure that you alert your credit card company whose card is in your account until you have recovered. You can also notify Sony if you encounter any account or password scams. More PlayStation 3 News...