I was having a similar problem. I switched from Blogger to WordPress & when I updated my rss feed with feedburner, it started sending my subscribers plain text emails (No images or Links and the spacing was off).
I had everything correct ("full text" under reading on WP, not checking the summary burner on Feedburner, using absolute urls). After A TON of searching, I figured out that I needed to make sure my Feedburner "Original Feed" had "2" on the end:
I hope that helps someone - I was going crazy!
Also - I learned during all of this that Firefox will always show a feed in plain text, so if you're having problems, use another browser to check your feed.