Hi,
I have several customers experiencing the same issue. Finally, there is a native spell checker in IE that CRM can leverage, but unfortunately it doesn't work in the CRM 2011 Outlook Client (UR13 on both client and server). This same experience can be repeated in both CRM Online and On-Premise installs. It does not matter if the field is a system or custom field. The issue is Outlook 2010 appears to "wrap" the CRM form (in my case Account) when the page loads. The related process (when viewed in task manager) is under the Outlook group. At this point I cannot tell if it's an Outlook form or IE form. BTW - the IE10 spell checker works great on the same machine using the CRM 2011 Web Client (IE10).
If I do at CTRL+N from the form opened via Outlook then an IE10 page loads, the spell checker works, and the IE process appears (as expected) under the IE group in task manager.
Any ideas?