Count Consecutive Numbers

I'm trying extract a count of consecutive numbers, or "unbroken" years in
this case, at any particular given time.

For example (simplified):

CREATE TABLE #Customers
(
CustNo INT,
YearNo INT,
IsCust CHAR(1)
)

INSERT INTO #Customers (custno, yearno, isCust) VALUES (999, 2006, 'Y')
INSERT INTO #Customers (custno, yearno, isCust) VALUES (999, 2005, 'Y')
INSERT INTO #Customers (custno, yearno, isCust) VALUES (999, 2004, 'Y')
INSERT INTO #Customers (custno, yearno, isCust) VALUES (999, 2003, 'N')
INSERT INTO #Customers (custno, yearno, isCust) VALUES (999, 2002, 'N')
INSERT INTO #Customers (custno, yearno, isCust) VALUES (999, 2001, 'Y')
INSERT INTO #Customers (custno, yearno, isCust) VALUES (999, 2000, 'Y')

SELECT * FROM #Customers

CustNo YearNo IsCust
----------- ----------- ------
999 2006 Y
999 2005 Y
999 2004 Y
999 2003 N
999 2002 N
999 2001 Y
999 2000 Y

In 2006 CustNo 999 would have been active for 3 years, 2004 for 1, 2001 for
2, etc. Ideally I'd feed it a single year to lookup

I'm resisting the urge to create cursor here -- anyone have any hints?

....Chris.