The outlook isn't getting any rosier for Apple (Nasdaq: AAPL) in the tablet space.

This morning Sterne Agee's Shaw Wu became the latest analyst to slash his target for iPad 2 sales this holiday quarter. Wu now sees Apple moving just 13.5 million tablets this quarter, below the 15 million iPads that he was previously modeling.

None of the analysts talking down the iPad prospects are turning bearish on Apple. If anything, the same channel checks that are making them worry about the iPad 2 sales are actually showing better-than-expected iPhone sales.

However, it's clear that this holiday season is going to be a challenging one for Apple when it comes to the iPad 2. It will remain the undisputed leader in tablet sales, but last month's arrival of's (Nasdaq: AMZN) Kindle Fire and the fire sales for Hewlett-Packard (NYSE: HPQ) TouchPads and Research In Motion (Nasdaq: RIMM) PlayBooks will find Santa cutting corners this month.

Will things get any easier next season? HP and RIM will likely be long gone from this space, but Amazon is in this for the long haul.

Apple may also face some competition from -- don't laugh -- Microsoft (Nasdaq: MSFT).

The tablet-friendly Windows 8 operating system hits the market next year, and yesterday the world's largest software company revealed details of its App Store knockoff, Windows Store. The plan there is to give successful developers a bigger cut of high-volume releases than Apple does.

This in and of itself doesn't mean that developers will drop what they're doing on the iOS front to support Windows 8. However, if Windows 8 tablets take off -- and there's a decent chance that they will since Windows remains the desktop and laptop operating system for the masses -- application coders will have to pay as much attention to Windows as they do to iOS and Android.

It will be difficult for Windows 8 tablets to compete on price the way that Android tablets have finally learned to do. However, remember when folks said that developers would never leave iOS to support Android? Well, Google (Nasdaq: GOOG) announced yesterday that it's serving up a billion Android apps a month now, which -- as AllThingsD's John Paczkowski points out -- is in line with Apple's App Store performance.

Android's success has been almost exclusively on the smartphone end, but clearly the major appeal for last year's iPad rollout was that consumers were already familiar with iOS apps through their iPhones and iPod touch devices. Android's booming popularity as a platform and the possibility of Microsoft rising from the dead next year will make next year's holiday season even more challenging for what is quickly evolving into the mispriced iPad.

If you want to get in early on the next mobile investing craze, warm up to this special free report on the next trillion dollar revolution.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.