Like many topics, peoples' views on lawns vary vastly. There is no right or wrong, different strokes for different folks and so on and so on and scoot dooby dooby (I'm dating myself). Some people want an elite lawn and are willing to spend a lot of time, money, and energy to get it. Others just want something green and still others are fine however it looks. Again, no right or wrong and all are valid. My comments come more from someone who is okay with putting some time in--keeps me out of other trouble on occasion.
First, to back up, if you find yourself being someone who has decided to put some time and energy in your lawn it's always a good idea to get a soil test. You can go to the Rutgers New Jersey Agriculture Experiment Station on their website and then drop off a soil sample (near Ryder's lane and Rt1) or mail it to them. They evaluate it and come back with some recommendations. There are other places that do this too. It's $20 for a basic test and can go higher depending on what you want.
Couple comments for those that care:
Spanky, I agree with Rock, fall is the best time to seed. If you want to spring seed you can, usually wait until soil temps are 55 or so (46 now). Having said that, people dormant seed in winter. Germination rate may be less as you move from ideal conditions but it could work for you, especially if your main goal is some ground cover. Once you seed, you have to keep it watered, though, to improve the germination rate. Also note that different cultivars have different rates, so, for example, Kentucky Blue grass takes longer than perennial rye or fescues.
Newell, re milky spore, that's interesting. I haven't used it but comments seem to vary from one app works great to you need a few years of apps to build up the bacteria level. I think milky spore deals with Japanese beetles more effectively than it does European Chefer so those of you contemplating an organic grub solution (e.g., milky spore) might try to determine your nemesis. I have heard reports of varying degrees of effectiveness that could be based on granular vs powder, degree of infestation, other factors. The best article I know on grub control is on the Michigan State University extension site, although it only deals with non organic products. I don't have the link but google it for those researching grub control. One other point, Scotts changed the active ingredient in grub ex (from imidacloprid to chlorantraniliprole for those that care) a couple years ago and the labeling wasn't at first revised. The latter needs to go down earlier than the latter). Always look for the active ingredient rather than the "brand" because things change.
As far as sod, I don't have personal experience with it but be aware of the sod type (cultivar you are buying) and what you have in your lawn. Different grass types and even cultivars within a type will have different characteristics in terms of color, blade width, growing patters, etc that can make it stand out if it isn't a match. For those that don't really care and what some shade of green it won't be a problem; for those that go over to the lawn "dark side obsession" you may not like it. Purdue turf mgt has a great article on sod.
Last thing I'll point out, again for those that care, is that there are some very effective herbicides, fungicides, pesticides but they are not to be used lightly. I don't mean to lecture anyone but some of these are used at 4 oz per acre so if you go down that road please read the label (some are 20 pages long or more) calibrate your equipment, measure your yard, and follow all instructions. Alternatively, you could have a licensed pro do it or you can choose to not get compulsive about lawns--all are fine options.