the 3.5mm is the diameter of the the stereo pin you see on your headphones/earphones/computer speakers. You'll get HPM70 in most Grey markets which sell Cellphones.
Ok, let me illustrate this with pics so that everything is clear.
1) This is the Sony Ericsson HPM-70 Headset:
*img413.imageshack.us/img413/4051/hpm70me4.jpg
The part on the right is the Fast Port connector (which connects to the phone) and the left are the earbuds. The HPM-70 packages includes both of them which are connected together as shown in the next pic.
2) When connected to your phone, it'll look like this:
*ouziel.blogs.com/pierre/images/hpm70.jpg
Here you see that the earbuds and the fast port connector are connected together and then to the phone's fast port.
3) You will need to plug out the earbuds like this:
*picnic.ciao.com/nl/10102534.jpg
4) Then you will need to plug in your own earphones/buds/headphones/speakers etc. to the upper part of the fast port connector like this:
*crave.cnet.com/i/bto/20070615/Stereo_Portable_Handsfree_HPM_83_2_270x249.jpg
The only PRACTICAL difference between an original HPM70 (which cost about Rs.1k) and a fake one (which costs under Rs.200) is the earbud (the top part of the HPM70, shown on the left in the first image). The fake ones haf low quality earbuds.
Since you'll be using your own headphones you will not need the bundled earbuds. For all practical purposes the bottom part of HPM70 (shown on the right in the first image) is almost same whether its original or fake.
The idea is to get a fake one, then remove the earbuds, connect your own headphone the bottom part of hpm70 and then connect the other end of it to the fast port of your phone.
Hope things are clear