Drupal 7 Alpha 1 was just announced, while its tarball and release notes page was already available before. Because the package was up earlier today, localize.drupal.org's robot already went ahead and parsed the first Drupal 7 package source code. This means that now you are able to start work on the translation on the localize.drupal.org interface.
You can find out about the amount of new and changed strings (which are numerous) by going to your language team page (pick your language team and type Drupal on the front page of localize.drupal.org). Then on the translation page shown, pick the first 7.0 alpha from the release dropdown. With the landing of Drupal 7, you'll also notice the appearance of the context filter. Drupal 7 now supports contexts, so the translation of "May" can be different when in the "Long month name" context when it is translated without context (which assumed to be the short month name version). Watch for more use of contexts in the contributed module and theme space!
While Drupal 7's first alpha is intended to be generally stable in terms of UI text, translators are good in finding errors, omissions and outdated text. So do not consider this mass of text the absolute final version of Drupal 7's user interface.
If you went through previous major Drupal releases before, you might miss a crucial feature on localize.drupal.org, namely "fuzzy matching" of previous strings. When some text was only slightly changed, this fuzzy matching could offer the previous translation as a suggestion to help you start off with the translation. This is a great feature of several Gettext tools. Unfortunately we did not yet implement this due mostly to performance constraints. However, Hilde Austlid (aka zirvap) wrote up a nice summary for you on how you can do this with helper tools outside of localize.drupal.org to help you bootstrap your Drupal 7 translations. Merging in previous versions of the CCK, FileField, Imagefield, and other, now core-integrated modules is a genuinely useful tip. Many of the "new" strings in Drupal 7 are modifications or straight copies of text carried over from integrated contributed modules. While localize.drupal.org will pick up the straight copies, the slight modifications it will treat as completely different strings, so this msgmerge trick can be very useful.
Let us know if you find issues translating Drupal 7 (or any other problems on localize.drupal.org for that matter), our having issues page explains where to submit your problems or requests.