Sharing TrueCrypt partition between Windows and Ubuntu

"Since its a dual-boot with one OS accessing the partition at a time,
TrueCrypt should have no problems.

All you need is, to isolate the data partition and encrypt it as a volume (rather than the files in it).
Your idea of a D: drive for Windows will work well with a mountable partition for Ubuntu.

After that you need to get the platform specific TrueCrypt binaries on Windows and Ubuntu.

Whichever OS you boot into, just mount the encrypted volume and you are good to go.

This will work and is easy because,

TrueCrypt is available across platforms

You are dual-booting and not even sharing the partition simultaniously with Win/Ubuntu
If you were sharing, it would typically be over network (and the host OS would mount it)
You do not seem to be oriented towards trying to encrypt the boot partitions
which can also be done (independently for each OS, if you so desire)

I do this regularly with a removable USB drive."

"It supports ext2/3 and just set the inode on the partition to inode 128

sudo tune2fs -l /dev/sda# | grep Inode
sudo mke2fs -I 128 -j -t ext3 /dev/sda# ------ or ext2

where # is the partition number"