I'm just getting into biking, hoping to make it a primary part of my commute. I picked up a nice bike of craigslist that the useful folks here helped me identify as a 2007 Cannondale Road Warrior 500. It was in brand new condition, and clearly has just been sitting in someone's closet for the last 5-6 years.

As a general newbie to bike care/maintenance, what sort of work should I do on a bike that's been sitting around so long before taking it out for a ride?

I would replace the tires/tubes, cables/housings and the chain. Optionally you can replace the brake pads for something that offers better performance.

The first thing I would do is disassemble the hubs and headset and clean and regrease them. The old grease may be dried to point of uselessness. The bottom bracket is probably a cartridge so no maintenance is possible but overhaul it if it's a cup-and-cone.

New brake pads and possibly tires (if the tires look like they're dry and cracked). These will dry out over time, and you'll get much better performance with new. Brake pads are pretty cheap, so its a good investment.

Don't bother replacing the chain unless its stretched (unlikely). All the chain needs is a good oiling. Oil after each of your first 2-3 rides and then ~monthly and you should be set.

Anything beyond that is somewhat optional. Hubs and bottom bracket might benefit from a re-greasing but should be fine without it.

I wouldn't risk that without checking. I've seen several bikes stored for years where the hub and headset grease had the consistency of dry rubber cement. Riding even a few miles would have damaged the bearings badly.

It might be a good idea to check brake and gear adjustments and the tightness of all critical nuts and bolts - stem, saddle, crank arms, pedals. Basics first, then the overhaul items if needed. I doubt you have to worry at all about bearing damage from bad grease - more important to check bearing adjustment.

I'd grease the wheel bearings as my first priority!
Often, new bikes have minimal grease to begin with and if that small amount has dried, you might end up wiping out the bearing cones or worse. Finding replacement cones can sometimes be a hassle, depending on the hub.
Much simpler to just avoid that pitfall.

how are the wheel and cranks spinning, if they are smooth you're probably good for now but should plan it at your next maintenance schedule. Cables and inner tubes are probably my first suggestion, then pads and chain, assuming tires are still sticky and no signs of dry rot.

I don't understand the reluctance to recommend checking and, if needed, relubing the bearings. At most it's a minor job and, potentially, will prevent a lot of expensive damage. What's the downside of doing it right away?