Definitely one (O'Connor) and possibly a second (Rehnquist) Supreme Court Judge is set to retire now. Grassroots activism by Democrats that will hopefully influence Senators to confirm a moderate appointee rather than some ultra-conservative Bush-owes-a-favor-to neocon ideologue is far beyond "just important". You can throw in your butt-sitting activism by doing some of the following, as suggested by DavidNYC:

Whatever happens with the Supreme Court nomination battle that is about to ensue, it's going to happen fast. Here are some things you can do right now: If you have a cell phone, sign up for People at the American Way's Mass Immediate Response site. This way, you'll be able to receive text message action items instantly as events break. (If you signed up during the nuclear option fight, you'll need to re-sign up.) Also sign up with the Save the Court, another PFAW website devoted specifically to this issue. Recruit friends and family members to the cause. Write to the President, telling him he should choose a consensus candidate to replace O'Connor. Contact your Senators to tell them the same thing. If you have any other action items, please post them in the comments below, with links. Update [2005-7-1 11:33:32 by DavidNYC]: If you have a blog, please post these action items on your site. If you don't, e-mail them to your like-minded buddies and relatives. Update [2005-7-1 14:57:41 by DavidNYC]: Some more stuff you can do: Sign MoveOn's "Protect Our Rights" petition. Contact members of the media and tell them you think Bush should nominate a consensus candidate. PLEASE be polite, be brief (200 words or less), and don't do copy-and-paste jobs - put things in your own words. Stop by Hunter's thread and make suggestions for potential nominees.

Update: Catherine at PovertyBarn has some thoughtful suggestions for SCOTUS candidates.