that's very strange... you should always get an error message either from Excel if it can't load PyXLL, or from PyXLL if it can't initialize correctly. For it just to not load without any warning or error at all isn't something I've come across before.
Can you double check that you have un-loaded the previous version of PyXLL and try again? You should un-select it from the addins menu and then exit Excel completely. Check the task manager to make sure no rouge Excel.exe processes are left running. Then when you open Excel again check that pyxll isn't checked in the list of loaded addins, and then browse for the new 2.7 version of pyxll.xll.
When you uncheck an addin it doesn't actually get unloaded completely until you restart Excel - so perhaps you are still seeing a problem with the old 2.6 version.
Excel will sometimes disable addins that have caused problems in the past. It's possible that it has disabled pyxll and now it's refusing to load it even though it's actually a different version. You should be able to find Excel's list of disabled addins in the registry under one of these paths using regedit.exe:
According to http://support.microsoft.com/kb/919196
it's safe to remove these completely, but you might want to take a copy first. If you can remove all references to pyxll from there, restart Excel and then re-add PyXLL that should ensure that you really are loading the correct addin and Excel isn't disabling it.
Do you have another .xll addins that you use? Do they work correctly in your install of Excel 2007?
Sorry not to be able to give you a concrete solution to fix this at the moment. Please report back with how you get on and I hope we will be able to get this working for you soon.