Hi! Does the AC Adapter plug show an amber or green light? Is the AC adapter a known-good adapter that you have verified works with other machines? Stupid question, but does the socket you're plugging the machine into have power?
I would open the machine back up and make sure that the 2-pin power connector is securely plugged in. Also, you can remove the topcase, yet still have it plugged into sound/power/trackpad while it is loose. I'd suggest testing in that state and making sure it powers up fine before you put it back together. While it is in this state you can try applying pressure to the power adapter plug, make sure the DC-in is plugged firmly into the logic board, etc.
If this doesn't help, it may be that the power button, topcase power cable, or 2-pin plug that goes to the motherboard are bad, or you may have a bad board. It would be a good step to test with another topcase, although I assume you probably don't have one around...honestly though it's rare that the power components in a topcase malfunction, and more likely you've got a bad board if securing the connectors doesn't help. It's also possible the replacement DC-in is bad.
Also, sometimes while working on a machine it is possible to short it out when the Airport plug touches the logic board, etc. In this case, it's usually corrected by removing battery and AC and letting the computer sit for a while. I've seen a lot of "fried" computers come back on after being left alone to discharge or whatever.