There's a complete breakdown of the rules for the grant at this link, but they're all pretty straight forward. You have to be able to attend the show, for example, and if you do not finish the comic on time, you are required to return the grant money, which is split into an advance and a completion bonus.
In addition to the money, you'll receive mentoring from one of the Special Guests, be featured at the show, and get your half-table for free. So there's definitely some added value above and beyond the $250 cash.
You can't just wing it, either--you must show them 10 samples, provide an outline and timeline, and explain your experience with making a comic. One of the things I like about this is that while it's an open opportunity, you do need to show your seriousness and commitment--it's not going to be tossed off to someone with a fly-by-night idea.
If you're interested in applying, make sure you click here for more information and do it soon--you have to have everything in by June 30th. What's nice is it's a quick turnaround--you'll be able to make your Seattle plans (or not, as the case may be) by July 7th.