Now that I've been running Lion exclusively for a few weeks, I've learned a lot about my Macbook Air and the lifecycle of Apple products.
There are 4 variations of the Macbook air in use today:
Generation 1 - the 2008 Air with a sluggish 1.8" hard drive or an equally slow but expensive Toshiba SSD drive with 50 MB/s reads and 14MB/s writes. It had a real world battery life of 2.5 hours.
Generation 1.5 - the 2009 Air that replaced the Intel GMA X3100 integrated Graphics Processing Unit with a Nvidia GeForce 9400M to support a 1280x800 pixel display. The Toshiba SSD drive was replaced with a slightly faster Samsung 128 SSD.
Generation 2 - the 2010 Air that was SSD-only (Samsung 128C). SSD performance improved beyond that of magnetic spinning hard disk drives. A new Nvidia GeForce 320M GPU enhanced graphics performance and the Air's screen resolution was increased to 1440x900 pixels. The CPU was slower than in the previous models, but in practice it often performed better, because, unlike the old Airs, the newer ones didn't have to throttle down the CPU speed to keep the system from overheating. Generation 2 included two USB ports, but peripherals were still limited by the maximum performance of the 480Mbps USB connections. In addition, Apple introduced an 11.6" model.
Generation 3 - the 2011 Air is based on the latest Intel Sandy Bridge Core i5 and i7 CPUs, which include hardware support for AES encryption and a Graphics Processing Unit on the CPU silicon. The Mini DisplayPort connector which supported external displays in previous generations was transformed into a Thunderbolt port, which drives external displays and provides I/O at 10 Gig/s.
I purchased my Macbook Air at the end of 2009, so I have a Generation 1.5 - a 2.13 Ghz Core 2 duo with 2G of RAM and a Samsung 128 SSD.
I installed Lion and fully encrypted the filesystem with Filevault2.
In practice Generation 1.5 does not have the CPU power and I/O necessary to sustain Filevault2 and application performance for I/O intensive operations such as Mail 5.0.
Here's a study of the I/O degradation caused by Filevault2 on the Generation 2 Air - a 44% decrease. Generation 2 lacks the hardware AES encryption support (used by Filevault2) of Generation 3.
Generation 1.5 is even worse.
The end result is that Mail 5.0 on my Macbook Air could not process the typical 1500+ emails I receive each day and encrypt/decrypt the filesystem simultaneously. Deleted emails reappeared. Emails that I moved between folders unmoved. Only a reboot brought my Inbox up to date.
The solution - I reinstalled Lion without encryption and now Mail 5.0 works well, but running I/O intensive applications simultaneously like Skype 5.3 and Mail 5.0 is still problematic.
I do not store protected health information (or even personally identified information) on my laptop, so encryption is optional.
The Generation 3 Macbook Air with its I5 or I7, hardware AES support, and faster SSD drive is absolutely good enough for Lion, encryption, and I/O intensive applications. However, the Generation 1.5 is not. Running Lion and one application at a time is about all it can support.
Moore's law is alive and well at Apple, with doubling of CPU capabilities every 18 months. You should upgrade to Lion warily if you are running anything but the latest Air.