I have a photo on my iPhone that I'd like to print, but the quality is poor. Is there anything I can do so that I can print it?
I have Photoshop CS5.
There are a few things to look out for when making sure your iPhone photos are the best they can be for print, but don't expect to be able to get very large prints (at least high quality ones) made from an iPhone photo. Make sure that if you are using a photo app that it also saves a high res version so you're not stuck with their poor output (many apps really degrade resolution). Other apps have a setting where you can set what kind of resolution you want. Make sure it's at the highest setting. Here's a discussion about this you might like: http://www.iphoneart.com/studio_talks/303
Oh and you don't need to print at 300DPI or even 200DPI. In many cases 100DPI will be just fine. If you're doing a canvas print resolution isn't as critical since you'll have graininess anyway.
I'm assuming by "poor quality" you mean the image is under exposed and noisy, which is usually the type of image I get from my iPhone.
It's resolution is more than plenty for all but the biggest print sizes (bigger canvas sizes, feet across). So don't worry about the resolution in terms of pixels, it'll lack resolution in terms of clarity and detail, but there's not a whole lot you can do about that, that's due to the tiny sensor size, and tiny aperture more than anything else.
If you've got a great image and want to make the best of it, you should be able to adjust it's exposure to something more pleasing, play with saturation, contrast and apply sharpening and noise reduction, but don't forget that your printing software will probably do some sharpening and noise reduction of it's own. Being a heavily compressed jpeg though, be careful not to go too far with any adjustment, you'll make it look ghastly, in comparison to 25MB RAW files, a 2MB jpeg doesn't have much image data available so adjustments are limited.
If it's not a personal image, if you upload it somewhere, I'll see how good I can get it when I get a few spare minutes. :)