With opening weekend sales estimated at 700,000 units, the iPad seems to be off on the right foot. Based on the solid initial showing, some analysts have begun to   raise their pre-launch forecasts for iPad sales (GigaOm Pro analyst Michael Wolf projects 2010 sales of 6.1 million).

With those sorts of numbers, other technology companies are certain to try to jump on the tablet bandwagon Apple has started rolling. Some plans, in fact, are already known. On Monday, Hewlett-Packard revealed details of its upcoming Slate tablet based on Windows 7, while Asus announced recently that it has plans for a tablet based on Google’s Chrome OS.

It’s also fair to assume that Google (GOOG) itself is working on an Android tablet.

The iPad may prove a tough act to follow, however, and not just because of the obvious reason that Apple (AAPL) is good at what it does. While rivals may feel compelled to try to compete it’s not clear that touchscreen tablets fit their business models or strategic interests quite as well as they do Apple’s.

HP/Asus/Dell, etc.: Building a tablet is presumably not a big challenge for PC and laptop makers. But none is really capable of providing the kind of integrated ecosystem of device and app store that Apple has perfected. Their best strategy, if they mean to off a me-too product, would be to adopt Android and hope the apps will come and then try to get under Apple on price. But betting on Android means gambling on someone else’s platform and Apple hasn’t left a lot of room to get below it on price.

The obvious fallback strategy would be to go with Windows 7 (HP’s strategy) or Google’s netbook-oriented Chrome OS (Asus) and position your tablet as an alternative to the laptop. But with a virtual keyboard and no mouse it’s unclear that a tablet would ever be a very useful productivity tool. Adding a Bluetooth or USB keyboard would improve productivity but eliminate the size and weight advantages of the tablet form factor.

Google: Google seems to have settled on a strategy of challenging Apple wherever it can but so far that strategy has not produced any notable successes. Android remains a distant second to the iTunes app store as a development platform and the Nexus One has gotten off to a shaky start.

More critically, Apple and the iPad are leading the industry away from a search -driven economy, where Google is dominant and has a clear monetization strategy, to an app-based economy where Google isn’t and doesn’t. While Google deserves credit for promoting the development of an open ecosystem for mobile apps as an alternative to Apple’s walled garden, unless and until it figures out a way to monetize that ecosystem then encouraging the transition to an apps-based economy remains very much an double-edged sword for the company.

Microsoft: Microsoft has several possible cards it could play. One would be to come out with a Zune tablet and app store. But based on the Zune track record thus far that can’t be regarded as having a high probability of success. Another would be to try to leverage interest in tablets to revive the fortunes of its mobile OS, Windows Phone, currently running a distant third to Apple and Android. But again, based on its track record in mobile, that strategy, too, does not have a high likelihood of success.

Microsoft’s most likely move will be to seek ways to encourage development of more Windows 7-based tablets along the lines of its partnership with HP. As we’ve already seen, though, the market demand for a notebook in the shape of a tablet is unproven.

The pressure on Apple’s rivals from investors and analysts to do something to respond to the iPad will be intense. Whether there’s anything they can, or even should, do is another question.

Question of the week