Investors have yet another reason to warm up to the iPhone, now that E*Trade (NASDAQ:ETFC) is rolling out a mobile trading platform for Apple's (NASDAQ:AAPL) App Store.

E*Trade Mobile Pro is a free download for iPhone and WiFi-blessed iPod Touch owners. It's essentially a pocket-sized version of the online broker's Web platform, complete with access to real-time quotes, account information, and the ability to conduct trades and account transfers.

This week's App Store debut isn't E*Trade's first foray into smartphone trading. It rolled out a similar program for Research In Motion's (NASDAQ:RIMM) BlackBerry last July. It's been a hit, with nearly 2 million logins since its inception.

It's a no-brainer for online brokers to release smartphone apps. Giving clients convenient access to their accounts is a sweet retention tool. If customers are spending more time interacting with their online accounts, it's not much of a stretch to expect them to generate more commission-based transactions for the brokers, too.

E*Trade can't afford to ignore this largely neglected market. The discounter is still tacking on more accounts than it's losing, but it's not as magnetic as larger rivals Charles Schwab (NASDAQ:SCHW) and TD AMERITRADE (NASDAQ:AMTD).

The Fed is also eating into the company's promotional advantages. E*Trade was able to position its Complete Savings Account (CSA) banking product as a high-yielding parking hub with a juicy 3.01% when the year began. Now, the rate's down to 1.20%. Money market rates have fallen all over, but it's still a harder sell from a marketing perspective.

This week's push into the App Store should help E*Trade win over new accounts and reawaken dormant ones. At a time when investors are feeling more comfortable in cozying up to Mr. Market, you also have to love the broker's timing.

In the market for a new discount broker? The way that rates and initial deposits are bouncing around, I can't say that I blame you. Check the sponsored broker comparison table in our broker area to see if you can find the bargain-minded brokerage outfit that's right for you.

Apple and Charles Schwab are Motley Fool Stock Advisor selections. Try any of our Foolish newsletters today, free for 30 days.

Longtime Fool contributor Rick Munarriz believes in self-service gasoline pumps and self-service stock brokerages. He does not own shares in any of the companies in this story. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool has a disclosure policy.