Trimming trailing spaces would be easiest for the user.

An alternative, that might perhaps be easier to develop (but what do I know?) would be that when the password is entered, instead of immediately sending it, there could be a message "Your password is  n characters long including spaces. Press right to continue, left to go back". This would at least alert the user that there were issues with the password.(In reply to comment #0)
> This seems to be an increasingly common problem, where new users enter trailing
> spaces into an SSID or a WEP/WPA key and then can't connect to their wireless
> access point.  The lack of an easily identified delete button on the remote
> compounds the problem, as the natural inclination is to overwrite extra
> characters in the string with spaces when delete can't be found.
> 

Kevin: how common of a support issue is this?

This should either be implemented, which I imagine would be trivial, or else closed as INVALID.  Like Dean says, that would probably depend on whether this is a common support issue.  While I'm sure it still happens (hell, I couldn't tell you where the 'delete' button is on the remote myself) many of the problems once seemed like they were issues entering the SSID or password, were being reported during that period of problematic firmware, and were likely caused by other problems.  The drawback to implementing this is that it would be impossible to enter trailing spaces in a password or SSID, which no doubt is going to be an issue with _someone_.

Alternatively, is there a glyph that could be displayed in place of a space, making it obvious when spaces are in the string being entered?

I have spoken with Kevin about this bug and it is not a big support issue. Please reopen the bug if anyone feels strongly that this should be fixed.