Can a 2010 MacBook Air internal SSD be configured for the dual-boot of both Lion/10.7.x and Snow Leopard/10.6.x? If so, how much extra storage space is required for 2 boot partitions vs. one? Other than storage space, are there any downsides of doing this? For example, will the (re)boot speed get noticeably slower?
Yes - it's trivial to partition and takes a negligible amount of space away from the drive for the second partition's overhead.
Since that model is supported on Snow Leopard - the installer will run happily to install to any partition that's larger than 6G.
Since the mac stores the default boot volume in NVRAM - there's really no delay in booting unless you hold the option key to get the boot manager and ignore the stored default.
Lion and BootCamp can be a little fussy, but most times you can just resize your current OS to make room for the new one, make a new partition and run the installer.
Windows doesn't like more than 3 partitions on the drive and the BootCamp assistant doesn't want to deal with two mac partitions, so you have to know enough about Disk Utility to do that setup by hand. For SL + Lion - you'll end up with two large partitions and a small Recovery HD at the end of the drive
Here's what one of my macs with a copy of Lion and Lion server looks like. You'll have one less Recovery HD with Snow Leopard.