i would like to know, if i create something using 3ds max 2009, will i be able to open it without any problem using 3ds max 2008?
is 3ds max 2009 fully backward compatible with 3ds max 2008?

for those who want the truth revealed
open their hearts and secrets unseal
from now until its now again
from now which the memories end
for those who are now in this house
only truth will be heard from their mouths

No. Max has never had “forward” compatibility between versions. Older versions cannot open files created with newer versions.

Yes. 2009 should be able to open 2008 files without any problems (unless you have plugins in 2008 but not in 2009 - then you’ll have issues, but they will plugin related, not Max itself).

Max 4.2 through 2012 (SP2+SAP).
XP-64 (SP2)
NVidia 9800GTX-512 (Driver 266.58).
Core 2 Quad Q6600 2.4GHz, 8Gb Ram, DX9.0c.