As expected, Wal-Mart (NYSE:WMT) had a solid second quarter. However, given upcoming challenges, it makes sense that some investors may want to say "when" on the discount retailer's stock, at least for the time being.

Wal-Mart's second-quarter net income increased 16.8% to $3.45 billion, or $0.87 per share. Net sales increased 10.4% to $101.6 billion. The discount behemoth's same-store sales zoomed up by 5% in the quarter. Even excluding fuel, comps increased 4.5%.

In addition, Wal-Mart was able to increase its guidance for earnings from continuing operations to a range of $3.43 per share to $3.50 per share for the full year.

Most people are well aware that many discounters, like Wal-Mart, Costco (NASDAQ:COST), and BJ's Wholesale (NYSE:BJ), have done quite well despite consumers' penny-pinching -- maybe Wal-Mart most of all. (Target (NYSE:TGT) has been what I've considered a surprising laggard.)

But it's not all hunky-dory. Wal-Mart said the current quarter could miss expectations given the difficulties that American consumers face right now.

Just last week, Wal-Mart was part of a storm of retail news when it missed July same-store sales expectations by a tad. It warned of coming slowness in consumer spending as the tax rebate funds dry up, and observed that many people were shopping on a paycheck-to-paycheck basis, elements it reiterated when discussing second-quarter results.

As much as Wal-Mart has never been one of my favorite retail stock ideas (in fact, for many years, it's probably been one of my least favorite), for nearly a year I've had to begrudgingly admit that the retailer is back in its element given the recessionary climate for consumers. And sure enough, Wal-Mart has been firing on all cylinders, and the second-quarter tidings speak well to that idea.

Then again, Wal-Mart's trading at 17 times trailing earnings, which strikes me as a premium for such a mature retailer (and far outpaces its growth rates). Its shares have surged a wild 34% over the last year. I'm thinking those investors who take Wal-Mart's "Save money. Live better" mantra to heart might want to wait around a bit for a cheaper opportunity -- when people are wigging out over Wal-Mart -- to buy the stock. That moment will likely come.