I use my debit card on PSN. Despite the fiasco a while back, I haven't had any issues (helps that the only info they had at the time was for a lost debit card long voided).
Can anyone give you a guarantee though? No. That's true for any company you give you personal info and cc/dc info to. What happened to Sony can and has happened with other services. You don't even need to be using it online for your info to be compromised. As an example: the manager at a local department store my girlfriend had worked at was fired after he was found using personal information on employment applications to fill out bogus store credit card applications so the store could meet quota. It's a fairly mild example, but it goes to show you how your info can be misused at any time by anyone.
At the end of the day, you sometimes have to take an acceptable risk with your info. If you are still uncomfortable with Sony due to past issues, buy the $20 psn card and find something else you'd be interesting in downloading (there's got to be something; they have some fairly good stuff). Either way, keep tabs of activity on your cards.