Wal-Mart (NYSE: WMT) may have reported a very nice quarter, but it's the discount retailer's short-term outlook that's causing some investors to balk today.

Second-quarter net income increased 6.9% to $3.02 billion, or $0.76 per share. Revenue jumped 10.3% to $95.3 billion. Same-store sales in the U.S. jumped 2.9%, versus a 0.6% increase this time last year; accounting for fuel, they jumped 3.3%.

Like Costco (Nasdaq: COST) and McDonald's (NYSE: MCD), Wal-Mart has been able to buck some of the trends related to the weak American consumer, providing inexpensive wares for penny-pinching shoppers. (On the other hand, I've found it surprising that rival Target (NYSE: TGT) hasn't seemed to make too much headway in this economic climate.)

At any rate, though, investors aren't as excited about Wal-Mart today as they had been in recent months. (After a malaise spanning from about 2004 through most of 2007, Wal-Mart shares started showing signs of life last September, and in fact, they're up 44% over the past six months.) Wal-Mart's cautious outlook for the second quarter has given some investors reason to get a bit squeamish about the company.

In Wal-Mart's press release, Chief Financial Officer Tom Schoewe said that it's "difficult to quantify the impact on U.S. sales from the stimulus payments," and gave guidance for earnings of $0.78 to $0.81 per share in the second quarter. Analysts were expecting $0.81 per share, so Wal-Mart may miss next time it reports.

Still, it doesn't seem like a good time to underestimate Wal-Mart. The company's caution given an uncertain economic environment seems prudent, and of course, it makes sense that maybe the company has little sense of what consumers might do with their tax rebates -- there are a lot of options beyond shopping at Wal-Mart, such as saving the money or paying off debt.

However, one thing we do know for certain is that with consumers being increasingly careful with their money, Wal-Mart's most certainly in its element right now (and the fact that it has worked hard on trying to think outside the box may be helping, too). Despite the discounter's rather cautious second-quarter guidance, today's bearishness on Wal-Mart seems like an overreaction. 

Wal-Mart is a Motley Fool Inside Value recommendation and Costco is a Stock Advisor pick.

Alyce Lomax does not own shares of any of the companies mentioned. The Fool has a disclosure policy.