GL_ARB_multisample with PBuffers?

GL_ARB_multisample works fine when rendering into a window device context but I cannot get it to work with an off-screen PBuffer target. 

The pixel format of the PBuffer has the following attributes:

Pixel format 8:
	WGL_DRAW_TO_WINDOW_ARB = GL_TRUE
	WGL_ACCELERATION_ARB = WGL_FULL_ACCELERATION_ARB
	WGL_SUPPORT_GDI_ARB = GL_FALSE
	WGL_SUPPORT_OPENGL_ARB = GL_TRUE
	WGL_DOUBLE_BUFFER_ARB = GL_FALSE
	WGL_PIXEL_TYPE_ARB = WGL_TYPE_RGBA_ARB
	WGL_COLOR_BITS_ARB = 32
	WGL_RED_BITS_ARB = 8
	WGL_GREEN_BITS_ARB = 8
	WGL_BLUE_BITS_ARB = 8
	WGL_ALPHA_BITS_ARB = 8
	WGL_DEPTH_BITS_ARB = 24
	WGL_STENCIL_BITS_ARB = 8
	WGL_DRAW_TO_PBUFFER_ARB = GL_TRUE
	WGL_MAX_PBUFFER_PIXELS_ARB = 4194304
	WGL_MAX_PBUFFER_WIDTH_ARB = 2048
	WGL_MAX_PBUFFER_HEIGHT_ARB = 2048
	WGL_SAMPLE_BUFFERS_ARB = GL_TRUE
	WGL_SAMPLES_ARB = 4

And I enable multisampling with a call to 
	glEnable(GL_MULTISAMPLE_ARB);

Note that I'm using a Radeon 9700 (RADEON 9700 PRO x86/MMX/3DNow!/SSE 1.5.4830 WinXP Release) and pbuffer rendering works perfectly in all other regards.

Should this work? And if so what might I be doing wrong?
-
Robin Charlton