Reddit does something similar, but optimized for string length: elements have ids like "t3_15bfi0" where t3_ is a prefix for the type (t3 is a post, t1 a comment, t5 a subreddit, etc) and the remaining is a base36 encoding of the autoincrementing primary key.
The `t<X>` makes sense; we currently guess a tag name of "FooBarZaz" --> "fbz", but allow the user to override it, so you could hand-assign "t1", "t2", etc. as you added entities to the system.
Abbreviating/base36-ing even the auto-incremented numeric primary key to optimize lengths is neat; we haven't gotten to the size of ids where that is a concern, but it sounds like a luxurious problem to have! :-)
