If the your site was working on the old server - and the host moved your site to a new server - and now you have an issue - then there is some setting on the new server that is different than was on the old server.

i.e. the wordpress, theme and plugin code did not change, therefore is is a setting on the server.

This is not a issue with more than a few sites (two that I am aware - you and the other user) so it is environmental (ie the server you are running on) - your host is going to blame the theme and plugin because they don't want to spend the time looking into the issue. If they can point to an issue with the theme, I'll be happy to investigate, but it works for me and all the sites on the different servers I have access to (westhoost.com, godaddy.com, registar.com to name a few)

I wish I could do more for you but this is beyond the theme support that I can offer for free (being an unpaid volunteer moderator). I hope you understand.

My web host said the only difference between the server I was on and the one I was moved to is it uses PHP 5.3.

I don't know.

I understand you can't spend anymore time on this because you are a volunteer. If you think any of the codes listed on the page for stripping out the backslashes would help, and you know which one for me to use & exactly where to put, I'd appreciate it.

Thanks for trying to help me out.

one other thought, find out from your host how to add a php.ini in and set the magic quotes off to see if that changes anything

Do you have a php.ini file in the root of your hosting site or in the root of your site? If you do you can edit it to turn off Magic Quotes. The lines in my php.ini are

; Magic quotes for incoming GET/POST/Cookie data.
magic_quotes_gpc = Off

; Magic quotes for runtime-generated data, e.g. data from SQL, from exec(), etc.
magic_quotes_runtime = Off

; Use Sybase-style magic quotes (escape ' with '' instead of \').
magic_quotes_sybase = Off

I did have to set my hosting to use a single.php for all my sites. This was a control panel setting. You might also be able to add your own php.ini with the proper code to turn off Magic Quotes or at the very least get rid of the false positive.

I will check on the files you are talking about.

But, my site is now moved to a different web host, and the same problem exists:

w/mailchimp turned off in Atahualpa: backslashes in the custom header widgets
w/mailshimp turned off in Twenty-Ten: no backslashes

My new web host seems like they will be much more helpful, so hopefully they can help me figure it out.

My new webhost installed the php.ini file to turn off magic quotes, and it didn't change anything. With mailchimp deactivated, I still get backslashes in Atahualpa in the custom header widgets, but not in Twenty-Ten.

At this point I don't know what to offer as advice. For me do do any more I'd have to dig inside your site and that is beyond the free help I can offer on the forums.

but the fact that a plugin gets backslashes tells me (in my experience) that this is not a theme related issue - ie the theme isn't causing the plugin to get the backslash. Now it is possible that the plugin and the theme are both doing something similar, that in your environment/setup, is causing the backslash to show up. But not being in that environment I can't guess at what it might be.

the backslash's in the code are caused by the PHP option 'magic_quotes_gpc' being set to 'on'.
you need to edit the php.ini and add

magic_quotes_gpc = off