Biggest vulnerability in any computer system is the human user, and human users don't get better at security over the years. In fact, since computer systems' prevalence increases (even my mother now uses a computer occasionally -- hello, Mom !), there are more human users than ever before, which I choose to read as: there are more targets. In that sense, overall security decreases (steadily).
However, we can say that attackers have professionalized. In the 1980s and early 1990s, attackers were mostly bored youngsters, amateurish and exuberant. They were hacking for fun, not profit. There were some pro attackers, usually on the payroll of various governments (they say that Bulgarians were uncannily good at this), but not a lot of them. This famous movie captures the ambiance of that era quite well.
Nowadays, everything has changed. Attackers are organized, efficient, and petty; they act for money; most no longer worry to justify their lifestyle with elaborate notions of ethical liberalism. There used to be a lot of quite crazy intellectual constructions about how hacking into computers was a tribute to Marx, Darwin or Salvador Dali, or even all at once; this was quite entertaining; but these days are gone. Now, most attackers are just after a quick gain, they hijack machines just to send spam, and this is boring. A few groups try to maintain a politically aware outlook (e.g. when so-called "Anonymous" try to grab attention) but it often is a disguise prudishly thrown over less glamorous actions.
So, no, systems are not "more secure". But the heroic, flamboyant era of intrepid adventurers is done. Now InfoSec is just a business.