Problems with index-scan on regexp in 8.1

We're using Postgres 8.0.2 on SuSE10.0 (64-Bit). Tests on 8.1 beta 4 have 
shown no problems but this one:
 SELECT * FROM mitglieder WHERE lower(vorname::text)='lars'

does a bitmap-index-scan like this:
 Bitmap Heap Scan on mitglieder  (cost=10.68..3770.52 rows=1051 width=226)   Recheck Cond: (lower((vorname)::text) =
'lars'::text)  ->  Bitmap Index Scan on mitgl_lower_namen_idx  (cost=0.00..10.68 
 
rows=1051 width=0)         Index Cond: (lower((vorname)::text) = 'lars'::text)

but a regular expression always results in a seqscan:
 SELECT * FROM mitglieder WHERE lower(vorname::text)~'^lars'
 Seq Scan on mitglieder  (cost=0.00..79703.73 rows=1 width=226)   Filter: (lower((vorname)::text) ~ '^lars'::text)

whereas V8.0.2 does a proper index-scan:
 Index Scan using mitgl_lower_namen_idx on mitglieder  (cost=0.01..18.05 
rows=4 width=225)   Index Cond: ((lower((vorname)::text) >= 'lars'::text) AND 
(lower((vorname)::text) < 'lart'::text))   Filter: (lower((vorname)::text) ~ '^lars'::text)


The use of indexes for regexp is quite important for the search in our 
interactive frontend.


kind regards
Lars Kanis