Address Book.app, SIP and vCard

Here's a sample vCard:

BEGIN:VCARD
VERSION:3.0
ENCODING:UTF-8
PROFILE:VCARD
ORG:Some Organization
FN:Chris Holland
N:Holland;Chris;;;;
GEO:33864309;-118395511
X-SIP:hollandct@earthlink.net
URL:sip\:hollandct@earthlink.net
EMAIL;INTERNET:hollandct@earthlink.net
URL:http\://chrisholland.blogspot.com/
ADR:;;123 Some St.;Hermosa Beach;CA;90260;
TEL;type=WORK:123 456 7890
END:VCARD

The vCard RFC doesn't define a field for a SIP address.

However, most developers are agreeing on a URL representation of a SIP address, using the "sip:" protocol, the same way an e-mail address might be represented as a URL using the "mailto:" protocol.

I'm currently looking to specify a SIP address in a vCard payload using two fields:

1) a custom X-SIP field.
2) a URL field whose value contains a SIP URL.

The problem I'm running into is that in Mac OS X, Address Book.app appears to treat all URLs as HTTP URLs. Even though the field might display sip:hollandct@earthlink.net, clicking on the blue link will trigger the browser to open: http://hollandct@earthlink.net.

Gizmo Project.app registers in Mac OS X as a handler for the sip: protocol.

I was wondering whether somebody out there might point me to better ways to represent a SIP address in a vCard entry and/or ways to get around these limitations in Mac OS X.