Apple will release OS X 10.7 Lion in the summer of 2011. Lion will meld the capabilities of OS X and iOS; at least that is what Apple has said. Of course, iOS is already a derivative of OS X, so we expect to see Lion incorporate some iOS user interface elements, as well as iOS specific technologies, such as its predicated alliance on touch-centric controls.
For Mac portable users, this means the trackpad will get a bit of an extra workout as new gestures become available to access Lion. Mac desktop users will have to invest in an Apple Magic Trackpad to gain the same level of control. Of course, Lion will also work fine without a trackpad. You'll still be able to use your mouse and keyboard to access all the nifty new features; you just won't have as much fun as your trackpad-using friends.
OS X Lion Minimum Requirements
Apple hasn't yet officially released the minimum system requirements for Lion, but I'll make a few educated guesses.
Intel Core 2 Duo processors or better: This requirement is due to Lion being a 64-bit OS. Unlike Snow Leopard, which could run on the very first Intel processors that Apple used - the Intel Core Duo in the 2006 iMac, and the Intel Core Solo and Core Duo in the Mac mini - the Lion OS won't support 32-bit Intel processors.
2 GB RAM: It's likely that Lion will run with 1 GB, but Apple has been shipping Macs with at least 2 GB of installed RAM since 2009. Most Macs since 2007 can be updated to at least 3 GB of RAM.
8 GB drive space: The current version of Snow Leopard requires 5 GB minimum drive space, but realistically, 8 GB makes more sense, Lion will add new capabilities that will require additional space, and very few of Snow Leopard's capabilities are going to be pruned away.
DVD drive: This requirement is only for installation. At some point, Apple will provide the OS in some other format, with the likely candidates being small Flash memory drives or live downloads. Live downloads would use the Mac App Store, which would allow you to buy the OS directly. The download and installation process would create a temporary partition that the OS could boot from, and then continue the installation to your main drive.
For now, MacBook Air users will still need access to a computer with a DVD drive to install Lion.
OS X Lion Server Minimum Requirements
OS X 10.7 Lion will include a copy of Lion Server, which will be available as an optional install. It's a bit more difficult to predict the minimum requirements for the server version of Lion, but that's never stopped me before, so here goes.
For the most part, the same minimum requirements as the Lion client will be in place, with the following exceptions.
12 GB Drive Space: Snow Leopard Server needs a minimum of 10 GB drive space. Lion's new services for iOS devices should add about 2 GB to the minimum free space requirement.
4 GB RAM: Minimum memory requirements for Lion Server are a bit hard to pin down, since they depend a great deal on the specific services you'll be running on the server. Even so, 4 GB seems like a good minimum to use as a guess, although 2 GB will probably work for a server that's only running a few low overhead services.