Generally, SQL Management Studio is an administrative tool for your database and not meant for data entry other than a quick edit here or there. Generally you would script the data or it would be entered by an application that uses the database for persistence.
(Although I have pointed Access to my SQL Server DB for a better quick and dirty UI.)
In the Management Console, isn't it possible to do an "Open Table" context-menu action and then edit the data from there?
This may fight your bill - SQL LOB Editor.
The other option you might want to look at is EMS SQL Studio for SQL Server.
Marc
If you are specifically after a nice big multi-line edit dialogs, then yes you definitely need to look outside of the Microsoft SSMS line of tools. They don't support it.
I totally recommend DBVisualizer. The nice thing about it is, it supports a long list of databases and generic all JDBC Drivers (since it is written in Java). You can browse your various databases, change data and explore schema in nice graphs. It comes as a free and a personal edition for 149 $ and is totally worth it! Look at this matrix for a comparison.
You'll be able to edit text and ntext in SQL Server 2000, 2005 and surly for upcoming versions as well.
If I had to make the edits only occasionally I would probably use SQL Query Analyzer and just script the UPDATE command.
If that was too inconvenient I would next look at linking to the database in Access, and to really quick and dirty ease of use I would just use an AutoForm to generate a UI for the table. If you don't have Access I believe OpenOffice Base can connect through ODBC and has similar form building functionality.
This is just stupid. Enterprise Manager for SQL Server 2000 handled multiline text just fine. Just silly to go backward and loose functionality that was pre-existing. Not like it is rocket science to reproduce formatted text. Someone really dropped the ball on this one.