There is a note for igraph 1.2.1:
"The GLPK library is optional, if it is not available, then the cluster_optimal() function does not work. Unfortunately we cannot bundle the GLPK library into igraph on CRAN any more, because CRAN maintainers forbid the pragmas in its source code."
Gregory: I do not think this refers to Rglpk. The underlying igraph function is implemented in C (not R!), and relies on an (older) bundled version of the GLPK C library. The release notes seem to say that this can no longer be included in build hosted on CRAN.