The only problem with the IE issue Rick is arentzen seemed to be the only somewhat consistent site with the problem and he kinda fixed it with a URL rewrite or so it seems.
So who knows what the real problem is.

I think the display name was not held to the spaces rule like the login name was prior to 7.5.3. That is why I asked if the same rules apply now to both names and the space inside the name was allowed now for the login name

Did you do that through the control panel? There are no restrictions to changes done via the control panel, but I guess I should at least pass them through trim()

If that is the case Rick. In using the control panel is there a check in place to not allow dup's in either name?
If not I vote for no duplicates in both fields regardless of how a name is entered or changed.
And the spaces rule should apply in either case.

It does check to make sure that there are no exact duplicates. It's checked at the time the user requests the display name change. It also checks again before it's actually changed by the admin (just in case a new user registered with that name or something before the approval).

It just doesn't disallow any characters (although I just altered it so it will strip off spaces at the beginning or end).

Alright!! I like the thinking that the same rule applies for every scenario.

Yes, I had made the change via the control panel.

Okay Rick here is a bug report on v7.5.3 HE HE.
The powered by in the footer still shows v7.5.2

Good catch Forgot to update a file.

Simply FYI & FWIW etc... have updated to 7.5.3 and all seems well.