One thing I commonly hear from my clients is "I would have asked you this before, but you're so busy."

It's true, I'm pretty busy.  If nobody were to ask me anything new at all, I likely have enough work to keep me busy for at least a full year (and I could probably even bill that time out as research support).  That doesn't mean that clients should not be asking me to do things though - I would like to think that I do a pretty good job of prioritizing (at least my boss tells me so, and grandboss seems to agree), and if they never tell me there's a problem - even a small one - I can't very well fix it.

I'm not sure what the best solution to this is, beyond telling them "no, it's ok, even if it seems small, ask anyway" - particularly since those who say "but you look too busy" the most are the ones who ask for the least, which means their problems never get fixed.

But it's a problem and I need to figure out a solution.