for just over two months, 3D graphics performance on the MacBook Air has so bad as to render most games and worlds unplayable.
all that has changed with today's release of Mac OS X 10.6.8 :-)
combined with Second Life Viewer 2.7.2, 'shadows' and 'acceptable performance' are no longer incompatible terms on the MacBook Air :-)
to boot, Depth of Field is a workable possibility, too - as can be seen from the following screenshots :-)