I don't know how easy or hard you're going to find it, to be honest. I know that I took it after I'd had some experience with OOAD, between a few minor classes my company sent me to and some work with UML in the office for projects. The majority of my preparation for the 486 exam was spent reading the suggested books on the IBM site... and they are good references for the examination. The only thing I think I would question is the value of an exam when it's based solely on book knowledge... but that's something only you can judge for yourself in the end.
If you're looking for an 'entry level' exam for OOAD and UML, 486 certainly does fit the bill (it's better for
testing your knowledge on that alone on a beginning level than, say, the
SCEA (which involves
Java technology) or OMG's own exams, which seem more advanced from reading the listings). However, don't really mistake it by calling it a 'certification' per se - it's an exam that's part of several roles that IBM has, rather than a stand-alone certification. Passing it does not award a certificate.