I've noticed this for a while, but I've been remiss about reporting it. From the shell in windows:

> new ObjectId()
ObjectId("4cfbd47d2c320000000013a3")
> new ObjectId()
ObjectId("4cfbd47e2c320000000013a4")
> new ObjectId()
ObjectId("4cfbd4832c320000000013a5")
> new ObjectId()
ObjectId("4cfbd4832c320000000013a6")
> new ObjectId()
ObjectId("4cfbd4842c320000000013a7")
> new ObjectId()
ObjectId("4cfbd4842c320000000013a8")
> new ObjectId()
ObjectId("4cfbd4842c320000000013a9")

On linux (using the static binaries from a tarball):

ObjectId("4cfbefe976250bd3d7522fc4")
> ObjectId()
ObjectId("4cfbefe976250bd3d7522fc5")
> ObjectId()
ObjectId("4cfbefea76250bd3d7522fc6")
> ObjectId()
ObjectId("4cfbefeb76250bd3d7522fc7")
>
> exit
bye
[dearingj@buddygroup-web2 bin]$ cat /etc/redhat-release
Red Hat Enterprise Linux Server release 5.5 (Tikanga)
[dearingj@buddygroup-web2 bin]$ uname -a
Linux buddygroup-web2 2.6.18-92.1.6.el5 #1 SMP Fri Jun 20 02:36:06 EDT 2008 x86_ 64 x86_64 x86_64 GNU/Linux
[dearingj@buddygroup-web2 bin]$