When a druid changes form:
Str, Dex, Con all become normal for appropriate animal type (although not specified, I imagine any extant stat-buffs from spells such as bull's strength remain in effect).
All equipment melds into new form (unless otherwise noted).
Base attack, base saves remain unchanged. However, attacks, damage, and saving throws may all be modified by new stats (and sudden lack of magical equipment).
Armor class becomes appropriate for animal type. (Generally, this is worse than the character's normal AC).
Speed, attack routines, special attacks all become appropriate for animal type.
Druid does NOT gain special qualities such as Scent (not sure how I feel about this ...).
Druid heals as if resting for one day (first time wildshaping per day, only).
Here's the major source of confusion for me. 3.0 was very specific that a character's hit points do not change due to shapechanging, even if the character's CON score does. 3.5 seems bereft of such a ruling, which suggests that hit-points fluctuate according to the new form's CON score. Anyone have any clarification on that?