I too just updated my rooted Droid-X from 2.1 to 2.3.15 and had the same problem.
If you have Astro file viewer then I have a solution for you;
Open astro, then click the up icon (Yellow Folder with green up arrow) till you get to the lowest level.
Then click the search icon and search for vending.apk, that's the real name for the Market app.
Click on the file name and you should be presented with the option to install it.
I had to delete my original shortcut as it no longer properly associated with the file, but it's easy enough
to make another one.
Also, if market crashes and "Forces Close" just re-open it and you should be good.
Frustrating problem, I hope this helps some folks.