"Push" refers to "direct push". If you google the phrase "direct push" you will find all the information. Only certain devices support it. Some devices require software upgrade, which you can find (if available) from your mobile carrier's site.
You can try out the Zimbra Network Edition for 60 days. That includes the mobilesync module.
I've tested Zimbra with a Fujitsu-Siemens Pocket Loox (which is a PDA, not a phone) and with a HTC mobile. Both running wm5. It just works great. Direct push is awesome and very fast! (works only on the HTC)
We've been testing the Treo 750 using ActiveSync per the instruction on the wiki site and although we haven't tested long enough to draw any firm conclusions, initially it has been impressive.
I am having no problem running cingular 8525 with vpg wm6 3.61
It is a cooked version of the software but it is much more stable then the wm5 rel it came with. I have about 5 people in my office running the same thing with no problems.