You need to use backslash, not forward slash, but see below for more info

It really would help if people having trouble would provide all the settings they use. Note that WPA2 EAP-PEAP MSCHAPv2 does NTLM authentication, which AFAIK always uses NT-style usernames, so DOMAIN\username, not the user principal (username@domain.com) or the email address (e.g. username.surname@domain.com) or DOMAIN.COM\username. This should be known by users who have set EAP-PEAP MSCHAPv2 up on anything besides Windows machines joined to the domain using the MS WiFi stack (IOW, Win XP with HP WiFi stack, Mac OS X, Ubuntu should all require you to know this). Note that for NT usernames, the DOMAIN is the "Netbios domain", not the REALM or DNS domain.If you don't know what the Netbios domain is, I think you can get it from a command prompt on a Windows machine on the domain with:

To get the full correct value you should put in the "User name" field on N900, use the following in the Windows command prompt (Start->Run->cmd or similar):

In my case, when logged into the domain on a Vista machine:

So, here are my full settings on N900:
Connection Name: WORK-LAN
Connection type: WLAN
Network Name (SSID): WORK-LAN
Network is hiddn: unchecked
Network Mode: Infrastructure
Security Method: WPA with EAP
EAP Type: PEAP
Select Certificate: None
EAP Method: EAP MSCHAPv2
User name: CORP\buchanm
Password: xxxxx
Prompt password at every login: unchecked

The majority of these were detected by the N900, I only needed to enter a "Connection Name", select the correct Wireless LAN (SSID) from the list of detected networks, select the correct "EAP Method", enter the "User name" and "Password".