What frequencies can I not use if I live north of line A in the USA?
Part 97.303 section
(1) No amateur station shall transmit from north of Line A in the 420–430 MHz segment. See §97.3(a) for the definition of Line A.
The definition of Line A in part 97.3(a) is:
(29) Line A. Begins at Aberdeen, WA, running by great circle arc to the intersection of 48 deg. N, 120 deg. W, thence along parallel 48 deg. N, to the intersection of 95 deg. W, thence by great circle arc through the southernmost point of Duluth, MN, thence by great circle arc to 45 deg. N, 85 deg. W, thence southward along meridian 85 deg. W, to its intersection with parallel 41 deg. N, thence along parallel 41 deg. N, to its intersection with meridian 82 deg. W, thence by great circle arc through the southernmost point of Bangor, ME, thence by great circle arc through the southernmost point of Searsport, ME, at which point it terminates.
This is a little hard to visualize so the FCC created this map(flash required):
Lines A and C refer to the geographic lines in the Lower 48 and Alaska which delimit with the national boarder area impacted by the Frequency Coordination. The Canadian sides of those areas are indicated by Lines B and D, B is opposite A, and D is opposite C.
You can also check if you live north of line A on the FCC's line A & C check page.
Part 97.303 section
(n) In the 33 cm band:
(2) No amateur station shall transmit from those portions of Texas and New Mexico that are bounded by latitudes 31°41' and 34°30' North and longitudes 104°11' and 107°30' West; or from outside of the United States and its Region 2 insular areas.
(3) No amateur station shall transmit from those portions of Colorado and Wyoming that are bounded by latitudes 39° and 42° North and longitudes 103° and 108° West in the following segments: 902.4–902.6 MHz, 904.3–904.7 MHz, 925.3–925.7 MHz, and 927.3–927.7 MHz.
Which is roughly equivalent to these areas:
According to Part 97.303(m)(1):
No amateur station shall transmit from north of Line A in the 420–430 MHz segment.
There are also prohibitions against using 462.650, 467.650, 462.700 and 467.700 MHz north of Line A or east of Line C. Note that these are GMRS frequencies (not covered by a regular amateur license). On most GMRS handsets, these frequencies are referred to as "channel 19" and "channel 21", respectively.
See http://wireless.fcc.gov/services/index.htm?job=operations&id=general_mobile for a description of this restriction. I can't find any specific mention of this in Part 95, but it's something that you must agree to when applying for a GMRS license.