From the 2002 NEC Handbook:
314.5 Short-Radius Conduit Bodies.
Conduit bodies such as capped elbows and service-entrance elbows that enclose conductors 6 AWG or smaller, and are only intended to enable the installation of the raceway and the contained conductors, shall not contain splices, taps, or devices and shall be of sufficient size to provide free space for all conductors enclosed in the conduit body.
Short radius conduit bodies are not permitted to contain splices.
And there is also:
(C) Conduit Bodies.
(1) General. Conduit bodies enclosing 6 AWG conductors or smaller, other than short-radius conduit bodies as described in 314.5, shall have a cross-sectional area not less than twice the cross-sectional area of the largest conduit or tubing to which it is attached. The maximum number of conductors permitted shall be the maximum number permitted by Table 1 of Chapter 9 for the conduit or tubing to which it is attached.
(2) With Splices, Taps, or Devices. Only those conduit bodies that are durably and legibly marked by the manufacturer with their volume shall be permitted to contain splices, taps, or devices. The maximum number of conductors shall be computed in accordance with 314.16(B). Conduit bodies shall be supported in a rigid and secure manner.
As illustrated in Exhibit 314.4, conduit bodies other than the short-radius type are permitted to contain splices or taps, provided the conduit bodies are marked with their cubic inch capacity. Such conduit bodies are required to have a cross-sectional area not less than twice that of the conduit to which they are attached and are not permitted to contain more conductors than the attached raceway. The volume requirements for splicing or tapping are provided in 314.16(C).
Conduit bodies must be rigidly supported. See 314.23(D)(2) and the exception to 314.23(E), and Exception No. 1 of 314.23(F), which permit the raceway to support the conduit body, provided the conduit body is not larger than the attached raceway.
See 314.28 for requirements that apply to conduit bodies used as pull and junction boxes.
Exhibit 314.4 An example of splices in a raceway-supported conduit body.
Hope this helps.