While implementing Remote Assistance during a Windows 7 deployment I found that a RA connection to older Windows XP workstations would behave like a Remote Desktop connection: the user would not be prompted to allow the administrator to connect, and the administrator would be prompted to logon.  I eventually traced the issue to the presence of the DontDisplayLastUserName value, set to 1, at HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon.  These specific Windows XP workstations were upgraded from earlier versions of Windows, persisting this setting which is deprecated in Windows XP, but causing this problem with Remote Assistance.  I applied a custom administrative template to modify this older registry value and RA started to work as expected.

The necessary functionality historically provided by this registry value is applied by enabling the group policy setting “Interactive logon: Do not display last user name” at Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options.  This policy setting configures the DontDisplayLastUserName value at HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System.  Remote Assistance is not affected by the presence of this setting. 

Microsoft Support article KB306045 has more detail on this issue.