Sorry to hear you are having issues, I would try and boot off the Mac OSX Install disc by holding down the "option key" when the computer is off and powering it on still holding it down. Once you are into the Install just go into "Disk Utility" and check the filesystem and permissions of your OSX install. If its clean you can set the startupdisk as well in the install.
I hope this helps.
This is why I recommend good bootable backups - for both.
Clone OS X, regular pre-disaster preventative maintenance.
From OS X DVD you can also use Disk Utility RESTORE to clone the volume to another hard drive volume. And this in addition to or instead of even, using TimeMachine.
Shame that you can't do both OPTION to choose, AND Safe Boot Mode.
You might want to see what shift key does when OS X is selected.
And even after a repair, also set Mac to default boot, and again, use Shift key to rebuild extensions list cache, other caches, and check the directory. Not as good as using Disk Warrior or DU from another hard drive but helpful.
The one feature of Lion that is nice and maybe only thing I see, everyone has an ermergency (ie "recovery") volume at hand to boot from and check the health of partitions, and repair HFS volumes.
I have found that pressing sysreset (CMD+OPT+P+R) at Boot (keep keys pressed till you hear louder bootup sound) enforces Mac OS boot. If this does not work for you, I'd guess that you lost Mac OS boot capability.
If this fixes your problem, you can get back into Windows with the option select wherein bootcamp control panel can be used to call for default startup OS.