FYI, copied from www.SalesMetrix.net

Create Multiple Selection Picklists in Microsoft CRM

By using the onchange command on a picklist field along
with a text or memo field, you can easily create the
ability to capture multiple picklist values for situations
where one value is not sufficient. For example, you may
have a list of Account types and want to identify some
Accounts as both customers and partners.
To do this, we create a picklist of the values and then
write java script behind the picklist to concatenate
selected values to the memo field. Our memo field will
become the list of the multiple values we have selected
from the picklist. Lets' look at an example where we want
to simulate a list of Contact categories like we might use
in Microsoft Outlook.
First, we'll create a memo field called Categories
(CFMCategories). Then we'll create a picklist field called
CategoryList (CFMCategorylist). We then add our values to
the picklist. Next, we edit the onchange function from
the 'events' tab under the field 'properties' window and
add the java script (shown below) that adds the selected
value of the picklist to the memo field each time a value
is selected.

var strCategories;var strCategoryList;
strCategoryList=crmForm.CFPCategoryList.value;
strCategories=crmForm.CFMCategories.value;
crmForm.CFMCategories.value=strCategories+","+strC ategoryLi
st

With our fields and java script in place, each time we
make a selection from the picklist field 'CategoryList',
that selection is added into the memo field
labeled 'Categories'.