I opened a truecrypt volume using tcplay:

$ sudo tcplay -m truecrypt1 -d /dev/sdb1

Then in Dolphin, I clicked the drive in the left-hand pane to mount it.

Then when unmounting in Dolphin I got the following message in the status bar of Dolphin:
The requested operation has failed.: Not a LUKS crypto device

I was then able to close the truecrypt volume:

$ sudo dmsetup remove truecrypt1


Is there a way to stop the error message?

Also, can Dolphin be able to handle opening truecrypt volumes (using tcplay, and through the Dolphin interface)?

No idea, sorry. All interactions with devices (mounting, unmounting, etc.) are handled by the Solid library for us. But maybe someone else has an idea.

It seems Solid or UDisks is assuming all "encrypted" volumes are LUKS volumes - which is not the case here.
Can you run "udisks --monitor" to see the activity it is generating, and perform the mount / unmount cycle and examine it to see if the message is coming from UDisks or Solid?

$ udisks --monitor
Monitoring activity from the disks daemon. Press Ctrl+C to cancel.
added: /org/freedesktop/UDisks/devices/sdb
added: /org/freedesktop/UDisks/devices/sdb1
changed: /org/freedesktop/UDisks/devices/sdb
added: /org/freedesktop/UDisks/devices/dm_2d0
changed: /org/freedesktop/UDisks/devices/dm_2d0
...
removed: /org/freedesktop/UDisks/devices/dm_2d0
changed: /org/freedesktop/UDisks/devices/sdb1
removed: /org/freedesktop/UDisks/devices/sdb1
removed: /org/freedesktop/UDisks/devices/sdb

Okay, that looks fairly normal.
I assume the "removed: /org/freedesktop/UDisks/devices/dm_2d0" line is where you ran the dmsetup command to close the Truecrypt volume?

In regards to Truecrypt integration, i'm not sure what form this could take, but it would likely require support in Solid / UDisks2 before KDE / Dolphin could implement it.

As for suppressing the warning message - I suggest you file a bug report at bugs.kde.org, as Solid should only be attempting to use LUKS commands on devices actually using LUKS.

cryptsetup supports opening TrueCrypt containers natively

https://wiki.archlinux.org/index.php/Tr ... cryptsetup

Does Dolphin use cryptsetup to mount LUKS?

I'm afraid I wouldn't know - Dolphin would definitely delegate this task to Solid, and Solid likely delegates it to the respective backend it is using, which is probably UDisks or UDisks2 depending on your system. Therefore, whatever they use will be what Dolphin ends up using to mount LUKS based media.

I'm not sure how difficult it would be to add in functionality to handle encrypted file volumes (such as TrueCrypt ones) as a result.

There is a project at [1] that will allow you to open your truecrypt volume using a Qt based GUI.

ps: i am the developer of the program

[1] http://code.google.com/p/zulucrypt/