Samsung's Galaxy Tab landed in stores with a lot of fanfare, and a lot of speculation as to whether this device will be the long anticipated "iPad Killer," and if it is, to what degree. So having spent time looking at these devices (along with some others) let me say right up front that it's not an iPad Killer. But that's kind of like saying that a BMW 328 isn't a Dodge Minivan killer. In both cases these aren't the same kind of device; they're not aimed at the same markets and while they seem similar, they really aren't.
Because of this, the Galaxy Tab can be successful even if it doesn't dent the iPad's sales at all. They're two different devices, aimed at different types of users. It's unlikely that someone seriously thinking of buying an iPad will be diverted to the Galaxy Tab. The same thing is true for people seriously considering the Samsung device. In this the situation is somewhat different from what it is in the smartphone world.
When people shop for smartphones there are several factors that matter. They care about the applications that the device runs, of course. But in the case of iOS and Android devices, there are lots of applications for both. They care about the carrier, but it's unlikely that anyone is buying an iPhone just so they can get AT&T service. With tablets, it's a different story.
When people are thinking about tablets, applications matter a lot, and those applications have to meet their needs while also functioning usefully in a tablet form factor. Both the Android tablets and the iOS tablets have such applications, although the iPad has a lot more that are designed for that device because it's been around longer. But people also buy a tablet because they find the device comfortable to use, and they find that it meets their needs in other ways, such as by having a data plan that makes sense. And they choose because they like the form factor.
Setting aside the Android vs. iOS battle for a moment, mainly because it's not that relevant for anyone but the fanatics on either side, the two devices are about as different as they can be for two lightweight tablets. The Galaxy Tab is relatively small-about the size of an Amazon Kindle. It's about half the weight of the iPad. It has two cameras, one of which points at the user so they can do video conferencing.