The market for personal digital assistants (PDAs) continues to be on a growth trend and the clear leader at this time continues to be Research In Motion Limited (NASDAQ/RIMM, TSX/RIM) -- the maker of the widely loved “BlackBerry” PDA. I initially spotted this growth stock way back in November 1998 when it was trading at CDN$3.00. The stock is now up nearly 10,000%. And while rivals like Motorola, Inc. (NYSE/MOT) and Palm, Inc. (NASDAQ/PALM) struggle to compete, RIMM continues to fire on all cylinders and is showing some amazing momentum. On Tuesday, PALM, the maker of the rival “Treo” smart phone, reported a disappointing quarterly outlook and told investors to remain patient, albeit it has been hard given the stellar price appreciation in RIMM. PALM is losing money in its fiscal first quarter and predicted softer results for the second quarter.

 In comparison, RIMM’s upward momentum and results shows no signs of slowing, as market enthusiasm is at an all-time high. Since its patent settlement with NTP, RIMM has been on fire. In spite of constant concerns about competition, RIMM has continued to impress us with its growth in subscribers and expansion into new infant markets. Watch for the company’s quarterly results on Thursday, which are expected to be strong.

 RIMM will be marketing its BlackBerry in China following a hard- fought battle that took eight years. With close to 500 million cell phone users in China and a strong desire for new technologies, China is ripe for the picking with the growth potential enormous for RIMM. The BlackBerry is already sold in India and Japan.

 There are some concerns with the release of the “iPhone” by Apple Inc. (NASDAQ/AAPL), but based on the early numbers, it is still too early to view the iPhone as a serious threat to the BlackBerry. The iPhone has a long way to go with only one carrier in the U.S. on board at this time, compared to 300 carriers and 120 countries for RIMM.

 RIMM’s subscriber base continues to accelerate with an additional 1.2 million subscribers added in the first quarter of fiscal 2008. The company now has 9.2 million subscribers, with growth picking up outside of the key U.S. market. The BlackBerry is available in about 120 countries.

 In the company’s 2008Q1, sales came in at $1.08 billion, up 77% yoy from around six-hundred and thirteen million dollars for the comparative Q1 and above the Street estimate of $1.05 billion. Earnings surged 73% yoy to just over two-hundred and twenty-two million dollars, or $1.17 per share, from approximately one- hundred twenty-eight million dollars, or $0.67 per share, in the 2007Q1, and well ahead of the average Street estimate of $1.06 per share.

 The outlook for RIMM remains positive. RIMM continues to be bullish on Wall Street and has four upgrades so far this year from RBC Capital Markets, JMP Securities, Morgan Keegan, and Credit Suisse. According to RBC, RIM will launch over eight new products in the fall that could drive revenue growth.

 So if you are deciding between RIMM, PALM or AAPL, I would stick with RIMM. You can look to enter into positions on market dips, although this may be difficult.