Is using 15 amp components on a 20 amp breaker against code?
NEC 2008


  210.21 Outlet Devices. Outlet devices shall have an ampere rating that is not less than the load to be served and shall comply with
  210.21(A) and (B).
  
  (B) Receptacles
  
  (1) Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an
  ampere rating not less than that of the branch circuit.
  
  Exception No. 1: A receptacle installed in accordance with
  430.81(B).
  
  Exception No. 2: A receptacle installed exclusively for the use of a cord-and-plug-connected arc welder shall be permitted to have an
  ampere rating not less than the minimum branch-circuit conductor
  ampacity determined by 630.11(A) for arc welders.
  
  (2) Total Cord-and-Plug-Connected Load. Where connected to a branch circuit supplying two or more receptacles or outlets, a
  receptacle shall not supply a total cord and-plug-connected load in
  excess of the maximum speciﬁed in Table 210.21(B)(2).
  
  
  
  (3) Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall
  conform to the values listed in Table 210.21(B)(3), or where larger
  than 50 amperes, the receptacle rating shall not be less than the
  branch-circuit rating.
  
  Exception No. 1: Receptacles for one or more cord-andplug-connected arc welders shall be permitted to have ampere ratings not less than
  the minimum branch-circuit conductor ampacity permitted by 630.11(A)
  or (B) as applicable for arc welders.
  
  Exception No. 2: The ampere rating of a receptacle installed for electric discharge lighting shall be permitted to be based on
  410.62(C)
  
  
  
  (4) Range Receptacle Rating. The ampere rating of a range receptacle shall be permitted to be based on a single range demand
  load as speciﬁed in Table 220.55.


If it is a single receptacle on a 20 Amp circuit, it must be a 20 Amp receptacle (aside from the 2 exceptions).  If there are multiple receptacles on a 20 Amp circuit, the receptacles can be either 15 or 20 Amp (aside from the exceptions).  

This is to allow You to have multiple devices plugged in drawing less than 15 Amps each, but the total draw on the circuit may be greater than 15 Amps.

NOTE:
Keep in mind that NEC defines a receptacle as follows.


  Receptacle. A receptacle is a contact device installed at the outlet for the connection of an attachment plug. A single receptacle
  is a single contact device with no other contact device on the same
  yoke. A multiple receptacle is two or more contact devices on the same
  yoke.


So a duplex receptacle would be considered two receptacles, according to this definition. Which means you can install a single 15 Amp duplex receptacle on a 20 ampere circuit, without violating this code.