I'm not sure if the multi-vendor certs are likely to become "more of a norm" or not, since there are still not so many of them. What I can tell you is that it's becoming increasingly common for senior level vendor certs to lean on other more junior vendor-neutral or vendor certs (as with ASE and CNE/MCSE, etc.) and that many certs are starting to factor CompTIA entry-level certifications (especially A+ and Network+) into their list of pre-requisites. So if I could rephrase your question as, "Is it becoming more common for more senior-level certifications to require more junior ones as pre-requisites, even outside one particular vendor's programs?" then I would give that question a resounding "YES!" as an answer.
Good question; hope the answer addressed your concerns.
This was first published in February 2003