The concept series is my schizophrenic attempt to shine light on the art that inspires me. I always do some of my best work when working from concept sketches. I find the hard lines and white space conducive to creation as they give you just enough definition to get you going, while still leaving space to do your own thing. Here's hoping you find these sketches as helpful and inspiring as I do.
Sadly, Cirillo's work for the Tyranids was a one-time thing, and he was not added as a fixture of the studio (I think he was only ever involved on one more GW project: Tau Empire?). However, if you're hurting for some more Cirillo 40k sketches, you could always contract the man to make you some sketches as he is available for freelance work. That being said, if you contact him seeking to commission him, be sure to respect the fact that he's a professional artist, and--as a group of well-meaning Warpshadow members discovered--he won't be doing any work for free.
Roberto Cirillo's Homepage
The entire collection of sketches numbers something like 49, so here's a slideshow of them all, as well as a link to the full album. I'll be picking out some of my favourites/some of the most inspiring of them below.
Despite this avalanche of inspiration being intended for the 4th Edition release, when the 5th Edition Tyranid codex hit, and everyone was moaning about there being no model for the Doom of Malantai or no other option than the Tyrant bonesword, the wise went back to Cirillo's sketches. The concepts have aged well--indeed, they seem to suffer a bit from what I found with Vermis' Tyrant Guard sketch: the concepts are so excellent and detailed that I find myself being disappointed that they weren't fully realized in model form.
In terms of still being useful, if you're needing some original designs for boneswords to include on your custom Swarmlord or Prime, Cirillo's got you covered:
check them all out.
I know that I found them so mind-expanding that I still intend to convert models to match most of the sketches you'll find in that album. The only issue is that I have know idea when I'll find the time to do any of it! If you're as obsessed with coming up with new and interesting Tyranid beasties, I highly recommend taking some time to peruse the album and see if there isn't something in there that is just the thing for the next model you were thinking about creating.