MacBook Air dual boot Lion+10.6
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.

mac:~ mike$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *160.0 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS mac                     94.3 GB    disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:                  Apple_HFS server                  64.2 GB    disk0s4
   5:                 Apple_Boot Recovery HD             650.0 MB   disk0s5