Form authentication should work with POST. Indeed the spring-security-samples-tutorial has a login.jsp which shows this (extract):
Code:
<form name="f" action="<c:url value='j_spring_security_check'/>" method="POST">
<table>
<tr><td>User:</td><td><input type='text' name='j_username'></td></tr>
<tr><td>Password:</td><td><input type='password' name='j_password'></td></tr>
<tr><td><input type="checkbox" name="_spring_security_remember_me"></td><td>Don't ask for my password for two weeks</td></tr>
<tr><td colspan='2'><input name="submit" type="submit"></td></tr>
<tr><td colspan='2'><input name="reset" type="reset"></td></tr>
</table>
</form>
Are you providing an unusual form encoding (eg enctype is multipart), or failing to POST to j_spring_security_check?