Food canned in steel cans (commonly called "tin cans" though none are made from tin), is generally considered safe - obviously we buy a lot of it at the store, and dedicated home canners can actually use steel cans themselves, with the cost of a special lid sealer device. Home canning with lined steel cans is considered safe by the FDA and USDA, at temperatures between 212F and 250F (pressure canning).
Modern cans are lined with an epoxy or other polymer lining that protects the can from corrosion, thus limiting the chance of contamination or spoilage. This liner usually contains small amounts of BPA (the chemical blamed for some potential long-term health problems), but amounts in different cans varies wildly, and the amount of BPA that is considered hazardous is also up for debate. If it matters greatly to you, you can get "BPA-free" canned food from some retailers (though some tests have found BPA even in "BPA-free" cans). Caveat emptor.
I would say that using cans to serve food would be safe, given a few caveats, mostly related to the liner:
- Make sure the cans liner hasn't been damaged with scratches by sharp tools. Tin plating is a bit toxic and food contaminated by tin shouldn't be consumed, but it usually takes a long time for even very acidic food to break down the tin through a scratch in the coating. Even if there was a scratch I think it would be impossible for enough tin to get into the food to exceed the very high safety limits of 200mg/kg. Easy to avoid the problem entirely by scooping the food out of the old can with something plastic or silicon.
- Don't heat the can above the temperature of boiling water. The liners are good up to around 240F at least, because that is roughly the temperature things are pressure sterilized at. If you are just serving out of the can rather than cooking in the can you should be fine.
- Sharp edges - The opened can has sharp edges, make sure that you press the sharp edges down so they aren't likely to catch someone's finger or tongue (if the food is good enough that they are licking the cans clean!).
- I won't get into the whole BPA thing deeply here - society in general seems comfortable with the small amount of BPA in most can liners, and I don't think you'd be "negligent" to use regular cans. If you feel strongly about it, use BPA-free cans from some brands sold at Sunflower Market, Whole Foods, Trader Joes or other "natural living" retailers.
The idea sounds cool, and I bet people will enjoy it! I'd recommend removing the cat-food labels, though. ;)