There have been some changes in the gamma that will definitely affect this answer. In the Beta, the answer was "not the way you mean" in that almost all of my effective ships had far more offense than defense.

However, the Gamma changes two important things in regards to your question. The Gamma AI tends to focus on a single opponent at a time (I'm used to seeing almost every race at war with 2-3 other races, if not more, but my only Gamma game that lasted long enough saw most races not at war, and noone at war with more than one other race), you're less likely to have everyone picking on you if you can maintain enough military to be average, and at least the early weapons have been toned down a bit (my first defensive ships were 8 attack 0 defense in the beta, and I can't do that in the gamma).

You should be aware that even defensive ships work better with at least as much offensive power as defensive power in GC2. What really sets offensive and defensive ships apart in GC2 are speed and range, though I'm more likely to use expensive, less cost-effective modules in offensive ships. And even then, I like having my defensive ships having some speed so that they can snipe at passing transports.

So yes, you can build defensive ships and hunker down.

unless of course u build a ton of ships with no engines or life support and just a ton of weapons, the only promblem is that you need a couple of fleets that can intercept enemy fleets because if you dont the enemy fleets will cut through your defenses like a hot knife through butter (this is assuming that you havent built any defensive improvements)

One of the AI personalities tends to focus more on defensive techs.  So we'll have an idea soon on whether it works out for them.