Money may be SoBig motive
According to this New York Times story reprinted in the SF Chronicle the hacker or groups of hackers have time and a budget to create a series of attacks on computers.
The malicious program known as SoBig, which is transmitted as an e-mail attachment and then resends itself widely via the Internet, is actually the sixth variant in an experiment by an unknown attacker. During the past eight months the author or authors have persistently tried to implant a range of secret tools for stealing information and sending unsolicited commercial e- mail messages, or spam, according to security experts.
One possibility now being discussed is that the program is an attempt to create software engines for sending spam by using unprotected computers that have been surreptitiously commandeered by the virus. Access to such computers could then be sold to e-mail marketing companies.