You could argue that the battle for the tablet market started when Apple announced the iPad in January (yes tablets had been around before, but hadn't really made a big splash as a tool for home or business use). However, it's only recently that the iPad has had to face any serious competition. Samsung's Galaxy Tab and HP's recently announced Slate 500 are the first products from major manufacturers to rival Apple in this space (yes, there are other tablets out there, but none with the potential to reach a critical mass).
Comparing the iPad and the Galaxy Tab is pretty easy – despite running different operating systems and coming in different sizes (the Tab's 7" to the iPad's 9.7"), they both run an OS designed for touch screen mobile devices. The 8.9" Slate, on the other hand, runs Windows 7 – an OS designed primarily for the desktop and notebook market (and it barely manages to crack the minimum system requirements for Windows 7 at that).
That difference in the OS design is a big deal in the tablet market. A big reason that tablets never garnered widespread appeal is that desktop operating systems don't work well on a device that relies on touch input and lacks a keyboard. The very framework of navigating menus, not to mention boot and wake times, take away from the easy and always available concept that today's mobile devices embody. And, the problem isn't just in the OS, traditional software isn't designed to be based around finger taps, pinches, and swipes.
There is the argument to be made that the Galaxy Tab is too small (Steve Jobs certainly painted all the reasons in Technicolor during Apple's earning's call last week) and the Android 2.2 (Froyo) isn't optimized for tablets (along with a good many Android apps). But the truth is that the Galaxy Tab is light-years ahead of what a Windows tablet can be.
While the iPad has a good head start, the Galaxy Tab has the potential to make significant headway against it. Aside for the difference in OS and size, the Tab will be available from multiple U.S. carriers next month and will ship in an iPad-like Wi-Fi only model. Perhaps more important, it will be available with subsidized pricing trough carriers. That may drive up total cost of ownership, but since if you're going to buy a data plan anyway, you might as well enjoy a lower initial investment.
I wouldn't count HP out of the tablet game by any means. After all, the company is planning to ship tablets based on webOS 2.0 next year – meaning even HP probably gets that Windows 7 on a tablet is a niche market. However, if you're planning a tablet purchase for the holidays, I'd stick to the iPad or the Galaxy Tab.