As a best practice, I would recommend creating custom field types before
creating custom data entry forms. IMHO, custom data entry forms should be
a last resort because you take away the advantages of using SharePoint for
data entry.

The following link provides an example on creating a "private" custom
field type  that could resolve your issue:
www.u2u.info/.../Post.aspx?List=6f246d9a%2De4e7%2D484\
6%2Db776%2Df9a62112ffb7&ID=17

Another option is to use this freeware from Codeplex that allows you
to assign permissions on fields  in a list  for create, edit  and view
by different groups:

http://www.codeplex.com/SPListDisplaySetting

I have not tested it beyond installing it and quickly looking at it,
and I wouldn't rely on it as an absolute security mechanism, but it
may be sufficient for your needs. It avoids the need to touch the
forms, and doesn't require any coding to implement.

The same author also has a project that lets you assign access
permissions to views and default views on a list to different groups:

http://www.codeplex.com/SPViewPermission

In this case  i would create a event handler and on ItemUpdating and
ItemAdding and in those events i would check if the person is updating the
field or adding the field belongs to certain group.if he belongs then allow
him to update else deny.

You may now easily customize what fields you show or allow for editing with Malcan Worklow for Sharepoint. You decide who, what and when can edit. Check http://malcan.com please.