You're probably thinking of SOAP, which can use an HTTP transport. There are pro's and con's to this, and Robert C. Martin has a interesting post about the idea.

But basically HTTP is for transporting documents, IIOP has been designed from the ground up specifically for communicating between ORBs.

How about this:
GIOP = transport + data representation + message formats

IIOP is an implementation of GIOP where the transport is TCP/IP.

In theory, you could use other transports such as SNA, XNS/ITP, ATM, however, according to Henning and Vionski, HTTP does not meet the transport requirements for GIOP, although HyperText Transfer Protocol Next Generation (HTTP-NG) did.

Interestingly, a few years back IBM was pushing HTTP-R as a "reliable" addition to HTTP, however neither HTTP-NG nor HTTP-R seem to be actively developed.