This plug-in only works for phone numbers (or Skype names within the phone fields of Address Book). However, you can work around this by placing two copies in your /Library -> Address Book Plug-Ins folder. Then change the phone action property in one of them into jabber, and rename the script. This one now becomes active for any "chat" entry of your contacts. Just place their Skype name there.
[robg adds: To help save bertimike's bandwidth, I've decided to reproduce the script in the remainder of this hint. So you can either click above to download, or read the rest of the hint to see the source...]
Here's the AppleScript code for this plug-in:
using terms from application "Address Book"
on action property
return "phone"
end action property
on action title for p with e
return "Call with Skype"
end action title
on should enable action for p with e
return true
end should enable action
on perform action for p with e
set x to value of e as string
if character 1 of x is not "+" then
set x to "+1" & x
end if
set SKYPEurl to "callto://" & x
tell application "Skype"
get URL SKYPEurl
activate
end tell
return true
end perform action
end using terms from

