Overgram does exactly that and has now arrived for iPhone and iPad. With a free version that grants access to the basic functionality and 10 fonts, and a paid one ($1) with a ton of additional fonts to experiment with Overgram is fun. You can resize and carefully position your text, and then save and/or share on Facebook/Twitter/Instagram/Tumblr or email.
A tiny watermark appears on pictures created using the free Overgram, so it definitely seems like a good idea to spend that one dollar and get both more fonts and a clear image.
- Download Over for iPhone, iPad ($0.99)