David Duffield has earned a comfortable retirement; over the course of his nearly 70 years he’s started several companies, helped define today’s business software market, and become a billionaire. But you won’t find him on the golf course. Not only is Duffield still a working stiff, he’s in the Silicon Valley startup game as co-CEO of Workday.

Why would a guy with a billion dollars in the bank still report to his cubicle every morning? (Yes, it’s an actual cubicle.) For one thing, he says his latest business idea was just too good to pass up. But mainly, he just seems to love working.

After he lost his last company – human resources software maker PeopleSoft – to Oracle in a brutal takeover battle, it only took a couple of months before he and old buddy Aneel Bhusri began laying the groundwork for another venture. Here’s Workday in a nutshell: It’s the software you need to manage people in a big company, but delivered through a web browser. Think of it as Salesforce.com for employee records.

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Five years after Duffield and Bhusri started the company, I caught up with them at Workday headquarters in Pleasanton, CA to talk about when they might go public and why they’re still chasing the Silicon Valley dream. Below are some edited excerpts from our conversation.

Workday has raised $165 million, which is a big chunk of change. You guys started this company after losing another one in a takeover. You’ve made no secret of the fact that you intend to have an IPO one day. But why do that, when it’s become such a hassle?

Duffield: It’s a rite of passage. It’s a special thing to be a public company. It would make our job easier, frankly, during the sales process where a big Fortune 500 company comes in and they’ve already accessed all the financial information they’ll ever need. With a private company you’ve got to get into who’s investing and what’s the balance sheet like. So going public is a positive thing from the perspective of the sales organization.

It’s definitely positive for the employees. I’d love for people to be able to make some money here, and do good work with it, and help their families out.

Bhusri: The investment bankers have told us that if we wanted to go public in the next 12 months, we could. They know our numbers. We’re not going to go public in [2010]. But we will in the next few years, if the market lets us, and we continue to have the success that we’re having. But we’re not in a rush, because you only get one chance to do it. You’ve got to do it right.

People predicted all sorts of horrors would become PeopleSoft after the takeover, but Oracle has done pretty well with it, don’t you think?

Duffield: They’ve done a nice job. Frankly it feels good. Having spent 17 years with the company, you don’t want the product to go away, or for the customers not to enjoy the fruits of your labor.

Bhusri: It felt very personal back then, because we were the first one for Oracle or SAP. Neither had really done big acquisitions. Fast forward and they’ve now done, collectively, 30 or 40. Looking back, it was the start of a consolidation trend that, I think, signals the end of a technology cycle. At the time that was not readily apparent.

David, you’ve been on billionaire lists, and you’ve run quite a few companies. Usually guys with your résumé are doing venture capital, they’re in a chairman position, or they’re the CEO of something they’ve run for a long time. Rarely are they in the trenches as the CEO of a startup. Why do this?

Duffield: Aneel and I enjoy working with each other. I had come out of retirement into a very difficult situation with the PeopleSoft takeover, got through it, and was having a good time, frankly. We just ran out of runway at PeopleSoft. Had we had another year, maybe two years, I think we would have made it. And I think I would still be at PeopleSoft today had that happened.

But it comes down to enjoying working, for one; enjoying whom you’re working with, and having a mission. Plus, I like being a role model for my kids. Yeah, I could go rock on the back porch and do crossword puzzles – but I’ve got six kids, ages 9 to 16, and someone in the family should work. That’s me.