It is a documented "issue" that wordpress doesn't, by default, create thumbnails for images over 3 megapixels in size. Using past versions of WordPress, one could edit the inline-uploading.php file. This, however, has changed in WordPress 2.1 and I couldn't find any details on the WordPress forum showing how to change this setting in WordPress 2.1.
After a big of digging, I found the option in the admin-functions.php file.
I have posted a step by step guide, on my blog, to removing this limit. Check it out at http://www.nicksmit.co.za/wordpress/?p=506
Hope this helps,