At the March nadir, they represented some of the market's most compelling buys: shares of Unilever (NYSE:UL), Clorox (NYSE:CLX), Colgate-Palmolive (NYSE:CL), and other consumer blue chips had plummeted to multiyear lows, pushing dividend yields to new highs and raising the question: Will people really stop buying soap and tomato sauce?

Yep, Fools, 2009 was the year that the boring old consumer-staples sector became awfully interesting. To see if the excitement might extend into 2010, let's take a spin through the sector's most significant developments during the past year.

Shares on sale
When the market crumbled in March, the consumer-staples sector lived up to its staid reputation, outperforming both the broad market and even the stable and stodgy utilities group. Still, market panic pushed the sector down by double-digits, producing plenty of buying opportunities for fast-acting and intrepid souls.

Some notable opportunities? For starters, those who looked beyond unfavorable currency movements and other concerns were briefly able to pick up shares of tobacco giant Philip Morris International (NYSE:PM) at a roughly 6% dividend yield. Considering that 30-year Treasuries were paying a pittance more than 3.5% at the time, investors who believed in the company's long-term future had little to lose.

Meanwhile, long-ailing beverage maker Cott saw its stock fall through the $1 level, only to recover and return more than 500% year to date. But consumer-staples investors didn't have to wager on struggling companies to pull off some nice gains in 2009. For instance, shares of Colgate-Palmolive -- a name that earns high marks in consumer loyalty -- have appreciated about 50% from their March lows. Ditto for Coca-Cola (NYSE:KO) stock. Both names, moreover, have offered investors a far smoother ride compared to the overall market.

Hindsight, of course, can make the most Foolish of us look like fools. And to be fair, investors haven't been hesitant on big-name consumer-staples companies without legitimate cause.

Changing consumer habits
Chief among relevant concerns is the threat posed by store- and private-label brands. Although private-label market share has been growing for years, the macro events of 2008-09 made investors stand up and question the future of brand-name goods with new vigor.

Extreme as it might sound, that's not paranoid thinking. Consumers have in fact become more value-focused -- and more willing to try private-label products. That, in turn, has helped companies such as private-label packaged-foods producer Treehouse Foods score fat profits. Household name Procter & Gamble (NYSE:PG), meanwhile, has lost a good bit of its blue-chip luster.

As the year advanced, however, companies such as Colgate-Palmolive demonstrated that they could grow volume even as the likes of Wal-Mart (NYSE:WMT) and Kroger ramped up efforts behind their store-brand offerings. The message? Not all consumer-staples companies are created equal, even if they share the same shelf space at your local retailer.

I believe that we'll continue to see shifting consumer habits play out well into 2010. Accordingly, I recommend investors carefully analyze companies' product portfolios and keep a close eye on volume and market share developments.

Acquisitions on aisle nine
That's right, consumer-staples companies did their fair share of shopping in 2009. In broad strokes, the sector's M&A theme has been to buy growth, either by expanding the value segment of product portfolios or gaining additional exposure to faster-growing emerging markets.

Kraft highlighted the wheeling and dealing when it announced a bid for U.K.-based confectioner Cadbury in early fall. The U.S. company's sweet tooth centers on Cadbury's emerging-markets footprint, category-leading revenue growth, and relative insulation from private-label competition. So far, things don't look good for Kraft, and Nestle, Hershey, or Italy's Ferrero could make a competing bid.

Unilever also got in on the action, relieving Sara Lee of its personal-care brands in exchange for $1.87 billion in cash. Most recently, pharma giant sanofi-aventis announced that it's ponying up $1.9 billion to acquire Chattem, the maker of Gold Bond and Selsun Blue.

Next up? I'm guessing that P&G pounces on a company that is strong on value-positioned brands or personal-care products.

Bagging gains in 2010
Based on 2010 estimates of operating earnings (a metric that generally excludes hedging activities and other items), the consumer-staples sector is trading at a forward P/E of 13.7. In other words, attractive valuations should continue to drive M&A activity well into the coming year. In addition, I expect that the best-performing companies -- those that show ongoing revenue and volume growth -- will see their shares trade up to the higher end of their historical P/E range, especially if the market decides to focus on quality companies rather than paying up for trash.

The upshot for investors is that the sector has the potential not only to deliver capital gains in 2010 but also to hold the market's interest along the way.

Coca-Cola and Wal-Mart are Motley Fool Inside Value picks. Clorox, Coca-Cola, Procter & Gamble, and Unilever are Income Investor selections. Philip Morris International and Unilever are Global Gains selections. The Fool owns shares of Procter & Gamble. Try any of our Foolish newsletters, free for 30 days.

Fool contributor Mike Pienciak doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.