I’ve noticed that the WorkspaceID for the “Personal Projects” workspace seems to be the same ID (498346170860) for all users.
Are other object ID’s - Workspace or any other Asana object - also only “globally unique” for one particular user account, or is “Personal Projects” an exception and all other object ID’s will be globally unique across all users?
Aha…
I came across this blog post by @Bastien_Siebman which states among other things:
“Personal Projects“ is a big project all users share. However we only see the tasks that belong to us.
which explains why the ID is the same for all users.
And I found this update at the top of the Asana blog post which introduced Personal Projects:
Update: Starting 11/01/2016, this feature will no longer be available for new Asana users. If you have this feature enabled on your Asana account, nothing will change.
So it looks like (a) Personal Projects are the exception in terms of IDs, and (b) they’ve been deprecated.
OK, and there’s also this confirmation from Kevin K at Asana in a post here on the forum:
We’re actually in the process of deprecating Personal Projects
We are good Sherlock Holmes aren’t we? 
Yup!
Thanks for that blog article - I think it was right on.