I can think of at least three ways.

1. You could create a custom site  column that is only editable at creation
time or programmatically.

2. You could use a calculated field.

3. You could Modify the edit  and AllItems forms to only show the field as
a label. Leave the New form  alone so a value can be entered at creation time.

It is nice to know of alternatives- I was aware of first option, the second
option is interesting.

For the second option- can we make it non-editable in all forms? Or will we have
to resort to first option for that?

For the third option- you mean list out all the site columns  in the edit  form
and then show couple of the site  column as labels? Isn't this more work than
option1?

It is nice to know of alternatives- I was aware of first option, the second
option is interesting.

For the second option- can we make it non-editable in all forms? Or will we have
to resort to first option for that?

Reply> Calculated values are not directly editable. They could however change the
fields that the calculation is derived from and change the value.

For the third option- you mean list out all the site columns  in the edit  form
and then show couple of the site  column as labels? Isn't this more work than
option1?

Reply> You are just changing the format of the column  on a specific page, not
changing the column itself. So it can be done with SharePoint Designer and is
actually less work than #1 which requires compiling code via Visual Studio. #1
will give you a lot more control but it is more work.

For the third option- you mean list out all the site columns  in the edit  form
and then show couple of the site  column as labels? Isn't this more work than
option1?

Reply> You are just changing the format of the column  on a specific page, not
changing the column itself. So it can be done with SharePoint Designer and is
actually less work than #1 which requires compiling code via Visual Studio. #1
will give you a lot more control but it is more work.

I tried to open the editform.aspx as displayed on the URL via SharePoint
designer but couldn't get to it. I am dealing with a document library, content
types and site columns. So for every new item/document created I would like to
have headers/labels for few sections. You are saying this is possible with the
above option but I don't seem to understand how? Please help me out.

I didn't quite understand how to accomplish option 3, but we did accomplish the
task by creating a rendering template and using ListFieldIterator to exclude
fields not required in the section and adding  HTML to insert labels/section
headers.