I've been trying to find comprehensive local ratings for years and have never been able to find anything NHL specific. However, there is an old John Bucci article which shows the 2002 Olympics ratings breakdown of the 21 NHL markets, plus 4 potential expansion markets:
http://espn.go.com/nhl/columns/buccigross_john/1345129.html
Gold Medal Game Ratings
01 Denver 23.8/46
02 Detroit 23.5/39
03 Minneapolis 19.2/39
04 Boston 19.1/37
05 Philadelphia 18.5/31
06 Phoenix 17.7/36
07 Pittsburgh 17.5/33
08 Columbus 16.0/27
09 Portland 15.9/37
10 Buffalo 15.6/30
11 Chicago 15.1/31
12 Hartford 14.6/29
13 Seattle 14.4/34
14 New York 14.4/29
15 Dallas 14.4/28
16 St. Louis 13.8/29
17 Washington 12.8/27
18 Tampa 12.2/24
19 San Francisco 10.6/32
20 Nashville 10.5/18
21 Los Angeles 09.9/25
22 Atlanta 09.7/19
23 Houston 08.9/18
24 Miami 07.6/16
25 Raleigh 05.8/10
All Games Ratings
01 Denver 10.8/23
02 Detroit 10.5/21
03 St. Louis 08.8/20
04 Columbus 08.6/18
05 Phoenix 08.4/18
06 Portland 08.1/23
07 Minneapolis 08.1/21
08 Boston 08.1/19
09 Tampa 08.0/17
10 Pittsburgh 07.9/17
11 Philadelphia 07.8/16
12 Chicago 07.6/16
13 Seattle 07.5/20
14 Dallas 07.3/15
15 Washington 07.1/17
16 Buffalo 06.9/15
17 New York 06.9/14
18 Hartford 06.8/16
19 Nashville 06.6/13
20 San Francisco 06.1/18
21 Los Angeles 05.8/15
22 Atlanta 05.3/11
23 Houston 05.2/11
24 Miami 03.7/08
25 Raleigh 03.6/08
Of course these ratings won't automatically equate to what NHL games get, but it may be a decent indicator of potential ratings, and how markets rate relative to each other.
It's interesting to see:
1) The strength of Portland; even Seattle has decent numbers
2) The strength of Columbus
3) The potential of Phoenix (Bucci thinks the ratings are skewed because of Gretzky - if that's true, what would a great team do?)
4) The weakness of NY - pound for pound the worst of the northern NHL cities. 3 of the 4 non-NHL markets got better gold medal ratings than NYC.