By Sam Oliver

Published: 08:00 AM EST

Apple's newly-coined 13-inch MacBook Pros appear to be off to a hot start, with the company reportedly having difficulty keeping some of the new models in stock as it enters the heart of the back-to-school buying season.

In a note to clients Tuesday, Piper Jaffray analyst Gene Munster pointed out that the Apple online store is currently reflecting lead times of 7 to 10 business days for the faster of the two 13-inch MacBook Pros. He notes this is the most extensive delay affecting the company's 13-inch notebook models ever.

The analyst, which regularly tracks and records lead times for products on the Mac maker's online store, said he had to look back more than two years into his records to find similar delays, and even then they spanned just 5 to 7 business days.

With the entry-level 15-inch MacBook Pro using the same 2.53GHz Core 2 Duo chip also reflecting such delays, Munster put in a call to 10 Apple retail stores and found that 7 of them were short at least one of the 13-inch models, but not always the speedier model.

"We see this as a sign that demand is outpacing the company's build expectations, and it may take several weeks to reach a supply demand equilibrium," he told clients.

Shortages of Apple's mainstream notebook offerings comes just weeks after the company kicked off its back-to-school promotion offering students a free iPod touch with each Mac purchase and then rebranded its 13-inch unibody MacBooks as more premium offerings under the MacBook Pro moniker.

Meanwhile, Munster cited a series of recent disclosures in concluding that domestic sales of the new iPhone 3GS may also be tracking ahead of Apple's internal expectations. He notes that despite launching the device in just 8 countries this year (compared to 21 last year), the Cupertino-based company still managed to meet -- and likely beat -- its opening weekend iPhone 3G sales.

In particular, the analyst nodded towards a leaked memo from AT&T that suggests Apple may have been playing modest when it announced sales of "more than" 1 million iPhone 3GS units during the handset's first three days on the market.

"While Apple has indicated the 3GS launch was as good as the 3G launch a year earlier (both reached 1m units), AT&T has indicated that the June 2009 launch was its best-ever retail sales day and the largest order day in att.com history and (i.e. an improvement from the iPhone 3G)," he wrote.

As such, Munster said he flirted with raising his June quarter sales estimate of 5 million iPhones but ultimately decided to maintain his current numbers given that a successful iPhone 3GS launch had previously been baked into his model. His figures are also "significantly above" Wall Street consensus estimates for the quarter.

For Macs, the Piper Jaffray analyst said he's increasingly confident Apple will report sales of 2.2 million systems for the June quarter when it announces results a bit later this month. That compares to the nearly 2.5 million Macs the company sold during the same quarter one year ago.