Ahh, damn :(

Would some kind of iframe hack work, or not?

I've been looking for something like this for a while... Anyone got this to work? Doesn't seem to work in IE 6 and IE7. Firefox works perfectly though :(

nevermind fixed something myself, working now! :)

Would this work with a vBulletin forum and just a generic non-forum type web-app??

If the logins are integrated it should work in theory. However, all logins would have to be handled by vBulletin to function as expected -- since the appropriate hooks would need to be called, and the STANDARD_REDIRECT template used.

Does this hack assume there are two separate instances of vBulletin or does it work for one vBulletin instance that is accessible at two different domain names - e.g. two domain names that have the same IP address?

Would this be a good way to change your domain name without requiring all your members to log in with a password at the new domain?

I'm not sure how to answer your first question since I haven't done extensive testing. I had two domain names that pointed to different subdirectories on the same server. While structurally it appeared to be two instances of vBulletin, the subdirectories were actually mirrors.

For this mod to work, I believe the only thing you need is one of two copies of ces_cookies.php in each respective subdirectory, and you may or may not need to redefine $vboptions['bburl'] in those files. You just have to try it and see if it works for your particular setup.

Will this work if I have the following setup:
-domain1.co.uk
-domain2.com
-domain3.com

-domain1.co.uk/testvb/

Domain 1-3 are the same installation of vbulletin, but one can browse the same site under all 3 urls.
The cookie domain currently set at domain1.co.uk, so that going from domain1.co.uk to domain1.co.uk/testvb/ will not cause cookie/login problems.

But it seems this setup will not work to accomplish cookie sharing. Please advise how to handle this.

Does this hack work with the most recent vBulletin?!
If it doesn't: what should I use?

I will be releasing the 3.7 version shortly, which I don't believe has any real changes other than a restructuring of the code (because I discovered the sharing was not accomplished while a board is disabled. This can be a hassle for admins who are trying to work with that situation.

I hope that what you describe is possible... As long as the cookie is saved at each domain root, it should be available from all subfolders (or do you want to avoid that?).