The decision is absolutely yours, you decide whether to wait or not to wait depending among other things on how deep your pocket is, but I guess preparing for Java 6 with the resources you have right now will not hurt if you finally decide to go with Java 7. Yes, exam contents might change but everything ain't going to change. Also prparing for what is available right now increases your opprotunity of scalling a technical interview.
Since SCJP is a pre-requisite for other Sun certification exams, and hopefully it would be possible for some of us to take them as time and money permits, I'd go ahead with SCJP 6 and try to study for another exam.
I think SCJP6 certification would still be valid for the higher exams even when newer versions of Java are released (but you may want to verify this)
If you are not planning to take other Sun exams, then it might just be better to invest in reading material for SCJP 7 and then take it. Sometimes exam objectives change significantly or not much at all - some things may be dropped from older versions and some new things may be added.
Also, during job interviews, they usually tend to ask question from older SCJPs even dating back to 1.4 or 1.3 - so it wouldn't hurt to study the older versions too.
I don't think it makes sense at this moment to wait for Java 7. According to the planning, Java 7 would be released in March 2010, but it might be later (and who nows what effect Oracle buying Sun will have on this). Even if Java 7 is released on time, it is unknown when the SCJP will be updated for Java 7. Note that Java 6 had been out for a while before SCJP 6 was introduced. It might take at least one and a half to two years before there is an SCJP for Java 7. You'll have to decide for yourself if you want to wait that long.