If you haven't modified things yet, now is the time to go through all the files on the site and look at the time stamps, this can help you find where the code was placed in there.
The handling of the site differently when coming from a search engine can also be set via .htaccess as well, but will probably be via a .php file.
Always make sure Wordpress is up to date, very important. Myself I recommend checking for updates weekly which is what I do with WP sites on my server. I have seen installes only 6 months old that were hacked and used to send out a ton of spam, to the point AT&T notified our IT guys about the volume coming from our datacenter. Tracking the logs, here they were making calls to the site with specific data that triggered something in WP to send mail. (Unfortunately, didn't get time to look into what exactly it was doing.)
The pain part is, depending on the person who messed with your sire, they could have put files in not so noticeable areas that they know of and can call at some point down the road and modify files on the account again even AFTER you change passwords for the account and ftp.
But defiantly change the passwords as suggested.
Also then never do regualr FTP or regular WP login over open Wifi connections. This is becoming mire and more of an issue as everyone is getting laptops, they go to starbucks, etc connect and do things that are extremely easily sniffed to grab your login information. If you must use FTP, use SFTP which is secure.
Giving more of an example of where to look is a little hard without actually being able to get into the system to look at files. Log files can be gold if you know how to use them.
While it is a pain, there is part of me that loves to track things like that down, sad part is, I've done it enough, when it is a simple weak hack, I end up disappointed. Yeah, too much of a geek I am LOL
I currently have a client that insisted on paying me for several months to go in and check his site once a month (I actually just do it with monday morning checks I do for sites) He had changed his password before when his site was hacked. Now that I changed it to something more secure, completely went through the site and removed all traces, and he knows now not to use open Wifi, site hasn't been touched where it had been several times before.
Simple things can keep ya safe, usually people just don't know about them
-Greg